Field of the Invention

The present invention relates to hydrostatic transmissions for a construction vehicle, and more particularly, to a closed loop hydrostatic transmission for a surface compactor providing reduced stopping distance and preventing engine overspeed during abrupt stop of the vehicle. Background of the Invention

Surface compactors, also known as road rollers, are mobile vehicles used to increase the density of soil and roadways and to seal and smooth asphalt surfaces. Such compactors are equipped with heavy drums and traveling relatively at low speeds. Thus, hydrostatic transmissions are typically chosen for their driving devices.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common knowledge in the field.

FIG. 1 shows a schematic diagram of a typical prior art hydrostatic transmission closed loop circuit 100 which includes a variable displacement hydraulic pump 101 driven by an internal combustion engine (not shown), and a fixed displacement hydraulic propulsion motor 102 connected to the drive train (not shown) of the vehicle. In a closed hydraulic circuit, pump 101 and motor 102 are fluidly coupled by a first conduit 103 and a second conduit 104. Pump 101 has a swash plate 101a the angle of which can be controlled either positive or negative from a neutral position. Changing the displacement of pump 101 by controlling the swash plate angle will change its output flow rate, which controls the speed of motor 102. Namely, the motor speed is zero when the fluid displacement of pump 101 is zero, which corresponds to a neutral position. Moving the swash plate angle negatively will reverse the flow out of pump 101, thus reversing the direction of motor 102. Depending upon the direction of the movement of swash plate 101a, the first conduit 103 (or the second conduit 104) of circuit 100 can be a high pressure supply line or a low pressure return line. Consequently, motor 102 is caused to rotate in forward and reverse directions, which corresponds to forward and backward travels of the vehicle.

A charge pump 105, also driven via the engine, supplies additional hydraulic fluid to closed loop circuit 100. Charge pump 105 draws fluid from a tank 106 and supplies this fluid into circuit 100 through a conduit 107 by way of one-way check valves 108 and 109 to compensate for any possible loss due to internal leakage. A relief valve 110 is used to provide a relief path to tank 106 so that more than the required flow from charge pump 105 cannot enter circuit 100.

Relief valves 111 and 112 are provided between conduits 103 and 104 and protect each conduit from pressure overload during operation. Relief valve 111 provides relief for conduit 103 and relief valve 112 provides relief for conduit 104.

Such closed loop hydrostatic transmissions provide dynamic braking during machine operation. Dynamic braking implies that the power from the vehicle's inertia is transmitted from the ground, through the wheels and final drive gears, to the motor, to the pump and back into the engine. All closed loop hydrostatic drive systems are subjected to this dynamic braking operation. Due to this dynamic braking operation, the vehicle will typically be encountered with a situation called an "overrunning operation", that is when the propulsion motor is not affected by means of the pump but is taken over by the vehicle's wheels. This condition can be created by the vehicle traveling down a grade or on flat ground and making an abrupt stop. In this case the wheel creates torque which drives the propulsion motor, which in effect becomes a pump. When this abrupt stop is initiated, the hydraulic pump connected to the engine is returned to a neutral swash plate angle which produces no flow and the pressure on the reverse side of the loop is increased. It is during this transition to the neutral position that the fluid being driven by the propulsion motor can regenerate horsepower back into the engine, which causes an increase in engine speed. Further, the closer the swash plate angle is to zero, the more likely that such engine overspeed increases. The consequences can result in reduced engine life or engine failure. Summary of the Invention

Technical Problems

The present invention has been developed with attention paid to these problems encountered in the above-described related techniques, and the objects thereof include providing a hydrostatic transmission for a construction vehicle which is capable of reducing engine overspeed during abrupt stops, and which attains an improved operation by reducing the stopping distance of the vehicle. Technical Solution and Advantageous Effect

In order to achieve the aforementioned objects, the present invention employs the following arrangement. According to one aspect of the present invention, there is provided a hydrostatic transmission for a construction vehicle which comprises:

a variable displacement pump driven by an engine;

a hydraulic motor connected to the drive train of the vehicle;

first and second conduits interconnecting said pump and said motor, said second conduit acting as a fluid supply path from said pump to said motor and said first conduit acting as a fluid return path from said motor to said pump;

a main relief valve with a relief pressure PI disposed between said first and second conduits, said main relief providing relief for said first conduit;

a selector valve disposed on said first conduit between said motor and said main relief valve, said selector valve being normally open and being shifted to close flow from said motor when activated;

a pressure switch disposed on said first conduit between said selector valve and said pump, said pressure switch sending an electrical signal to activate said selector valve when a set pressure P2 has been reached;

a brake assist relief valve bypassing said selector valve, said brake assist relief valve being designed to open at a relief pressure P3 to allow flow from said motor; and wherein the relief pressure PI of said main relief valve is higher than the sum of said set pressure P2 of said pressure switch and said relief pressure P3 of said brake assist relief valve. It is preferable that said selector valve is shifted between a first position allowing both flows from said pump to said motor and from said motor to said pump and a second position closing flow from said motor to said pump. Said selector valve normally maintains the first position and is shifted to the second position when activated by the electrical signal from said pressure switch. More preferably, a one-way check valve allowing flow from said pump to said motor and preventing flow from said motor to said pump is provided for the second position of said selector valve.

It is also preferable that said brake assist relief valve is lying on a bypass conduit branched off from said first conduit at a motor side point of said selector valve and joined with said first conduit at a pump side point of said selector valve. Said relief pressure P3 to open said brake assist relief valve is provided from the pressure at the motor side of said first conduit.

It is also preferable that said second conduit is the conduit that becomes a high pressure supply line when the motor is caused to rotate in the forward direction, which corresponds to the forward travel of the vehicle.

If an abrupt stop of the vehicle is initiated while the fluid discharged from the pump was being supplied to the motor through the second conduit, the speed control is rapidly pulled to neutral and the pressure on the reverse side of the loop, i.e., in this case the pressure of the first conduit, is increased due to the dynamic braking of the hydrostatic drive system. When the pressure of the first conduit exceeds the set pressure P2 of the pressure switch, then the pressure switch closes a contact and sends an electric signal to the selector valve. This signal activates the select valve to be shifted to the second position which closes flow from the motor, and forces the fluid to flow across the brake assist relief valve. When the select valve is activated, the resistance to flow is additive and the motor will then see a pressure corresponding to the sum P2 + P3 of the set pressure P2 of the pressure switch and the relief pressure P3 of the brake assist relief valve. That is, the pressure at the motor side of the brake assist relief valve can climb to a pressure of P2 + P3, while the pressure to the pump side thereof is maintained at a pressure corresponding to the set pressure P2 of the pressure switch. Also, when the pressure at the motor side exceeds a pressure of P2 + P3, then the brake assist relief valve is opened and the pressure to the pump side starts to increase but is always lower than the pressure at the motor side by the amount of pressure corresponding to the relief pressure P3. This reduced pressure to the pump during an abrupt stop will reduce engine overspeed. Additionally, the motor will see a pressure of PI + P3 which is the sum of the relief pressure PI of the main relief valve and the relief pressure P3 of the brake assist valve when the pressure to the pump side is reached to the relief pressure PI of the main relief valve. This increase in pressure, i.e., added resistance to flow, seen by the motor will reduce the stopping distance of the vehicle.

According to another aspect of the present invention, there is provided a hydrostatic transmission for a construction vehicle which comprises:

a variable displacement pump driven by an engine;

a hydraulic motor connected to a drive train of the vehicle;

first and second conduits interconnecting said pump and said motor;

a first main relief valves with a relief pressure PI disposed between said first and second conduits, said first main relief valve providing relief for said first conduit;

a second main relief valve with a relief pressure Ρ disposed between said second and first conduits, said second main relief valve providing relief for said second conduit; a first selector valve disposed on said first conduit between said motor and said first main relief valve, said first selector valve being normally open and being shifted to close flow from said motor when activated;

a second selector valve disposed on said second conduit between said motor and said second main relief valve, said second selector valve being normally open and being shifted to close flow from said motor when activated;

a first pressure switch disposed on said first conduit between said first selector valve and said pump, said first pressure switch sending an electrical signal to activate said first selector valve when a set pressure P2 has been reached; a second pressure switch disposed on said second conduit between said second selector valve and said pump, said second pressure switch sending an electrical signal to activate said second selector valve when a set pressure P2' has been reached;

a first brake assist relief valve bypassing said first selector valve, said first brake assist relief valve being designed to open at a relief pressure P3 to allow flow from said motor;

a second brake assist relief valve bypassing said second selector valve, said second brake assist valve being designed to open at a relief pressure P3' to allow flow from said motor;

wherein the relief pressure PI of said first main relief valve is higher than the sum of said set pressure P2 of said first pressure switch and said relief pressure P3 of said first brake assist relief valve; and

wherein the relief pressure Ρ of said second main relief valve is higher than the sum of said set pressure P2' of said second pressure switch and said relief pressure P3' of said second brake assist relief valve.

It is preferable that each of said selector valves is shifted between a first position allowing both flows from said pump to said motor and from said motor to said pump and a second position closing flow from said motor to said pump. Each of said selector valves normally maintains the first position and is shifted to the second position when activated by the electrical signal from its respective pressure switch. More preferably, a one-way check valve allowing flow from said pump to said motor and preventing flow from said motor to said pump is provided for the second position of each of said selector valves.

It is also preferable that said first brake assist relief valve is lying on a bypass conduit branched off from said first conduit at a motor side point of said first selector valve and joined with said first conduit at a pump side point of said first selector valve. Said relief pressure P3 to open said first brake assist relief valve is provided from the pressure at the motor side of said first conduit. Likewise, said second brake assist relief valve is lying on a bypass conduit branched off from said second conduit at a motor side point of said second selector valve and joined with said second conduit at a pump side point of said second selector valve. Said relief pressure P3' to open said second brake assist relief valve is provided from the pressure at the motor side of said second conduit.

It is noted that said first conduit becomes a high pressure supply line when the motor is caused to rotate in one direction, for example, a forward direction which corresponds to the forward travel of the vehicle. At this time said second conduit becomes a low pressure return line.

If an abrupt stop of the vehicle is initiated while the fluid discharged from the pump was being supplied to the motor through the second conduit for traveling in one direction, the speed control is rapidly pulled to neutral and the pressure on the reverse side of the loop, i.e., in this case the pressure of the first conduit, is increased due to the dynamic breaking of the hydrostatic drive system. When the pressure of the first conduit exceeds the set pressure P2 of the first pressure switch, then the fist pressure switch closes a contact and sends an electric signal to the first selector valve. This signal activates the first select valve to be shifted to the second position which closes flow from the motor, and forces the fluid to flow across the first brake assist relief valve. When the first select valve is activated, the resistance to flow is additive and the motor will then see a pressure corresponding to the sum P2 + P3 of the set pressure P2 of the first pressure switch and the relief pressure P3 of the first brake assist relief valve. That is, the pressure at the motor side of the first brake assist relief valve can climb to a pressure of P2 + P3, while the pressure to the pump side thereof is maintained at a pressure corresponding to the set pressure P2 of the first pressure switch. Also, when the pressure at the motor side exceeds a pressure of P2 + P3, then the first brake assist relief valve is opened and the pressure to the pump side starts to increase but is always lower than the pressure at the motor side by the amount of pressure corresponding to the relief pressure P3. This reduced pressure to the pump during an abrupt stop will reduce engine overspeed. Additionally, the motor will see a pressure of PI + P3 which is the sum of the relief pressure PI of the first main relief valve and the relief pressure P3 of the first brake assist valve when the pressure to the pump side is reached to the relief pressure PI of the first main relief valve. This increase in pressure, i.e., added resistance to flow, seen by the motor will reduce the stopping distance of the vehicle. Brief Description of the Drawings

FIG. 1 is a schematic hydraulic circuit diagram showing a conventional hydrostatic transmission;

FIG. 2 is a side view of the entire configuration of a surface compactor; and

FIG. 3 is a schematic hydraulic circuit diagram showing a hydrostatic transmission according to one embodiment of the present invention;

Detailed Description of the Invention

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. While the present invention will be described in conjunction with the following embodiments, it will be understood that they are not intended to limit the present invention to these embodiments alone. On the contrary, the present invention is intended to cover alternatives, modifications, and equivalents which may be included within the sprit and scope of the present invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, embodiments of the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. FIG. 2 illustrates the schematic configuration of an entire surface compactor. As shown in FIG. 2, a compactor 1 is composed of a front drum 2, a rear drum 3, a front body 4 and a rear body 5. A driver's seat 6 is mounted either on front body 4 or on rear body 5. Two bodies 4, 5 are connected with each other by means of a pivot joint 7 so the vehicle can be steered. Rear drum 3 can be replaced by wheels in certain applications.

FIG. 3 shows a schematic diagram of a hydrostatic transmission closed loop circuit 10 according to one embodiment of the present invention. Referring to FIG. 3, the hydrostatic transmission closed loop circuit 10 has a variable displacement & reversible pump 11 driven by a prime mover (not shown) such as a diesel engine or an electrical motor, and two fixed displacement hydraulic motors 12a, 12b connected to the drive train (not shown) of the vehicle. Pump 11 and motors 12a, 12b are fluidly coupled by a first conduit 13 and a second conduit 14.

Pump 11 can be an axial piston pump of which the fluid displacement can be determined by a tilted angle of swash plate 11a. By changing the angle of swash plate 11a, the stroke of the pistons can be varied continuously. If swash plate 11a is perpendicular to the axis of rotation, i.e., a zero angle, no fluid will flow and the motor speed is zero. If it is at a sharp angle, a large volume of fluid will be pumped and the motor speed is increased.

Pump 11 allows swash plate 11a to be moved in both directions from the zero position, pumping fluid in either direction without reversing the rotation of pump 11. In other words, the swash plate angle can vary from a zero angle, i.e., a neutral position, to a positive angle and to a negative angle. Consequently, motors 12a, 12b are caused to rotate in a forward direction and a reverse direction, which correspond to the intended forward or backward travels of the vehicle. It should be noted that two motors 12a, 12b are not necessarily required. It is possible to use only one motor in certain applications.

Depending upon the direction of the movement of swash plate 11a, the first conduit 13 (or the second conduit 14) of circuit 10 can be a high pressure supply line or a low pressure return line. In this embodiment, it is assumed that the first conduit 13 becomes a supply line and the second conduit 14 becomes a return line when motors 12a, 12b rotate in forward directions, i.e., when the vehicle travels forward.

A charge pump 15, also driven via the engine, supplies additional hydraulic fluid to closed loop circuit 10. Charge pump 15 draws fluid from a tank 16 and supplies this fluid into circuit 10 through a conduit 17 by way of one-way check valves 18 and 19 to compensate for any possible loss due to internal leakage. A relief valve 20 is used to provide a relief path to tank 16 so that more than the required flow from charge pump 15 cannot enter circuit 10.

First and second main relief valves 21, 22 are provided between the first conduit 13 and the second conduit 14 and protect each conduit from pressure overload during operation. The first main relief valve 21 provides relief for the first conduit 13 and the second main relief valve 22 provides relief for the second conduit 14. The first main relief valve 21 has a predetermined relief pressure PI to open the path from the first conduit 13 to the second conduit 14, and the second main relief valve 22 has a predetermined relief pressure Ρ to open the path from the second conduit 14 to the first conduit 13. In this embodiment, relief pressures PI and Ρ of both main relief valves 21, 22 are set to 5,500 psi.

Circuit 10 has a first selector valve 23 disposed on the first conduit 13 between motors 12a, 12b and the first main relief valve 21. The first selector valve 23 is normally open and shifted to close flow from motors 12a, 12b when activated. To be more particular, the first selector valve 23 is shifted between a first position 23a and a second position 23b. The first position 23a allows both flows from pump 11 to motors 12a, 12b and from motors 12a, 12b to pump 11, and the second position 23a closes flow from motors 12a, 12b to pump 11 but allows flow from pump 11 to motors 12a, 12b. The first selector valve 23 normally maintains the first position 23a and is shifted to the second position 23b when activated. Preferably, a one-way check valve 23c is provided for the second position 23b to allow flow from pump 11 to motors 12a, 12b and to prevent flow from motors 12a, 12b to pump 11.

A first pressure switch 24 is disposed on the first conduit 13 between the first selector valve 23 and pump 11. The first pressure switch 24 sends an electrical signal to activate the first selector valve 23 when a predetermined set pressure P2 has been reached. In this embodiment, the set pressure P2 is set to 4,000 psi. The first selector valve 23 is shifted to the second position 23b when the electrical signal from the first pressure switch 24 is received. Circuit 10 has a first brake assist relief valve 25 which bypasses the first selector valve 23. The first brake assist relief valve 25 is designed to open at a predetermined relief pressure P3 to allow flow from motors 12a, 12b. To be more particular, the first brake assist relief valve 25 is lying on a bypass conduit 26 branched off from the first conduit 13 at a motor side point of the first selector valve 23 and joined with the first conduit 13 at a pump side point of the first selector valve 23. The relief pressure P3 of the first brake assist valve 25 to open the path from motors 12a, 12b is provided from the pressure at the motor side of the first conduit 13. In this embodiment, the relief pressure P3 is set to 1,250 psi.

A second selector valve 26 is disposed on the second conduit 14 between motors 12a, 12b and the second main relief valve 22. The second selector valve 26 is normally open and shifted to close flow from motors 12a, 12b when activated. To be more particular, the second selector valve 26 is shifted between a first position 26a and a second position 26b. The first position 26a allows both flows from pump 11 to motors 12a, 12b and from motors 12a, 12b to pump 11, and the second position 26b closes flow from motors 12a, 12b to pump 11 but allows flow from pump 11 to motors 12a, 12b. The second selector valve 26 normally maintains the first position 26a and is shifted to the second position 26b when activated. Preferably, a one-way check valve 26c is provided for the second position 26b to allow flow from pump 11 to motors 12a, 12b and to prevent flow from motors 12a, 12b to pump 11.

A second pressure switch 27 is disposed on the second conduit 14 between the second selector valve 26 and pump 11. The second pressure switch 27 sends an electrical signal to activate the second selector valve 26 when a predetermined set pressure P2' has been reached. In this embodiment, the set pressure P2' is set to 4,000 psi. The second selector valve 26 is shifted to the second position 26b when the electrical signal from the second pressure switch 27 is received. Circuit 10 has a second brake assist relief valve 28 which bypasses the second selector valve 26. The second brake assist relief valve 28 is designed to open at a predetermined relief pressure P3' to allow flow from motors 12a, 12b. To be more particular, the second brake assist relief valve 28 is lying on a bypass conduit 29 branched off from the second conduit 14 at a motor side point of the second selector valve 26 and joined with the second conduit 14 at a pump side point of the second selector valve 26. The relief pressure P3' of the second brake assist valve 28 to open the path from motors 12a, 12b is provided from the pressure at the motor side of the second conduit 14. In this embodiment, the relief pressure P3' is set to 1,250 psi.

It should be noted that the relief pressures PI, Ρ of main relief valves 21, 22 are not necessarily set to 5,500 psi, and likewise, the set pressures P2, P2' of pressure switches 24, 27 and the relief pressures P3, P3' of brake assist valves 25, 28 are not necessarily set to 4,000 psi and 1,250 psi, respectively. Modifications for these pressure settings are available according to the requirements from the hydrostatic transmissions and the vehicles in use.

However, it is preferable that the relief pressure PI of the first main relief valve 21 should be higher than the sum of the set pressure P2 of the first pressure switch 24 and the relief pressure P3 of the first brake assist relief valve 25. In this embodiment, the relief pressure PI (5,500 psi) is higher than the sum of the set pressure P2 (4,000 psi) and the relief pressure P3 (1,250). The above condition also applies to the relief pressure Ρ of the second main relief valve 22, the set pressure P2' of the second pressure switch 27, and the relief pressure P3' of the second brake assist relief valve 28.

The following description is now provided about the operation of the hydrostatic transmission described above.

In the normal traveling operation of the vehicle, a fluid discharged from pump 11 is supplied to motors 12a, 12b through the first conduit 13 for traveling in one direction (or through the second conduit 14 for traveling in another direction), and is returned to pump 11 through the second conduit 14. The fluid flows across the open path of the first selector valve 23 which was maintaining its first position 23a. When motors 12a, 12b are under some load, an increase of pressure in the first conduit 13 occurs. If this increased pressure exceeds the set pressure P2 (4,000 psi) of the first pressure switch 24, then the first pressure switch 24 closes a contact and sends an electric signal to the first selector valve 23. This signal activates the first select valve 23 to be shifted to the second position 23b which still allows the fluid to flow to motors 12a, 12b. When the overload is large enough to reach the relief pressure PI (5,500 psi) of the first main relief valve 21, the fluid from one port of pump 11 is returned to the other port of pump 11 through the first main relief valve 21. No more fluid is supplied to motors 12a, 12b.

If an abrupt stop of the vehicle is initiated while the fluid discharged from pump 11 was being supplied to motors 12a, 12b through the second conduit 14 for traveling in one direction, the speed control is rapidly pulled to neutral and the pressure on the reverse side of the loop, i.e., in this case the pressure of the first conduit 13, is increased due to the dynamic braking of the hydrostatic drive system. When the pressure of the first conduit 13 exceeds the set pressure P2 (4,000 psi) of the first pressure switch 24, then the first pressure switch 24 closes a contact and sends an electric signal to the first selector valve 23. This signal activates the first select valve 23 to be shifted to the second position 23b which closes flow from motors 12a, 12b, and forces the fluid to flow across the first brake assist relief valve 25. When the first select valve 23 is activated, the resistance to flow is additive and motors 12a, 12b will then see a pressure corresponding to the sum P2 + P3 (5,250 psi) of the set pressure P2 (4,000 psi) of the first pressure switch 24 and the relief pressure P3 (1,250 psi) of the first brake assist relief valve 25.

That is, the pressure at the motor side of the first brake assist relief valve 25 can climb to a pressure of P2 + P3 (5,250 psi), while the pressure to the pump side thereof is maintained at a pressure corresponding to the set pressure P2 (4,000 psi) of the first pressure switch 24. Also, when the pressure at the motor side exceeds a pressure of P2 + P3 (5,250 psi), then the first brake assist relief valve 25 is opened and the pressure to the pump side starts to increase but is always lower than the pressure at the motor side by the amount of pressure corresponding to the relief pressure P3 (1,250 psi). This reduced pressure to pump 11 during an abrupt stop will reduce engine overspeed.

Additionally, motors 12a, 12b will see a pressure of PI + P3 (6,750 psi) which is the sum of the relief pressure PI (5,500 psi) of the first main relief valve 21 and the relief pressure P3 (1,250 psi) of the first brake assist valve 25 when the pressure to the pump side is reached to the relief pressure PI (5,500 psi) of the first main relief valve 21. This increase in pressure, i.e., added resistance to flow, seen by motors 12a, 12b will reduce the stopping distance of the vehicle. Likewise, if an abrupt stop of the vehicle is initiated while the fluid discharged from pump 11 was being supplied to motors 12a, 12b through the first conduit 13 for traveling in another direction, the speed control is rapidly pulled to neutral and the pressure on the reverse side of the loop, i.e., in this case the pressure of the second conduit 14, is increased due to the dynamic breaking of the hydrostatic drive system. When the pressure of the second conduit 14 exceeds the set pressure P2' (4,000 psi) of the second pressure switch 27, then the second pressure switch 27 closes a contact and sends an electric signal to the second selector valve 26. This signal activates the second select valve 26 to be shifted to the second position 26b which closes flow from motors 12a, 12b, and forces the fluid to flow across the second brake assist relief valve 28. When the first select valve 26 is activated, the resistance to flow is additive and motors 12a, 12b will then see a pressure corresponding to the sum P2' + P3' (5,250 psi) of the set pressure P2' (4,000 psi) of the second pressure switch 27 and the relief pressure P3' (1,250 psi) of the second brake assist relief valve 28. That is, the pressure at the motor side of the second brake assist relief valve 28 can climb to a pressure of P2' + P3' (5,250 psi), while the pressure to the pump side thereof is maintained at a pressure corresponding to the set pressure P2' (4,000 psi) of the second pressure switch 27. Also, when the pressure at the motor side exceeds a pressure of P2' + P3' (5,250 psi), then the second brake assist relief valve 28 is opened and the pressure to the pump side starts to increase but is always lower than the pressure at the motor side by the amount of pressure corresponding to the relief pressure P3' (1,250 psi). This reduced pressure to pump 11 during an abrupt stop will reduce engine overspeed.

Additionally, motors 12a, 12b will see a pressure of Ρ + P3' (6,750 psi) which is the sum of the relief pressure Ρ (5,500 psi) of the second main relief valve 22 and the relief pressure P3' (1,250 psi) of the second brake assist valve 28 when the pressure to the pump side is reached to the relief pressure Ρ (5,500 psi) of the second main relief valve 22. This increase in pressure, i.e., added resistance to flow, seen by motors 12a, 12b will reduce the stopping distance of the vehicle.

It should be noted that the hydrostatic transmissions according to the present invention are not limited to surface compactors shown in the embodiments, but can be widely used for various road machinery including, soil compactors, asphalt compactors, and asphalt pavers.

Industrial Applicability

The present invention provides hydrostatic transmissions for a construction vehicle which is capable of controlling and reducing engine overspeed by reducing pressure to the pump while dynamic braking operation is being performed due to an abrupt stop. Also, the increase in pressure, i.e., added resistance to flow, for the propulsion motor during an abrupt stop will reduce the stopping distance of the vehicle, and improve vehicle operability and safety.

Although the invention has been described with reference to the preferred embodiments in the attached figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.