GB2039638A - Hydro-mechanical variable-speed gear - Google Patents

Abstract

In an epicyclic gear including a first axial shaft 6 coupled to a planet carrier 7, a sun wheel 9 coupled to a second shaft 11 co-axial with the first and an annulus 12, one or more screw- type hydrostatic pumps 13 are driven by the annulus, while a plurality of screw type hydrostatic motors 14 are driven by the planet carrier, and control valve means selectively controls the supply of fluid to the motors from the pump(s), a pressure relief valve being provided in the fluid circuit. By selective operation of the motors through the control valve means, a variety of speed ratios can be chosen. This transmission system does not require a torque converter, and considerable operational cost savings should arise where the system is to operate at reduced speeds for considerable periods. The system could serve as either a step-up or a step-down transmission by having the input through the first shaft 6 or the second shaft 11 respectively. <IMAGE>

Description

SPECIFICATION Power recovery for variable speed drives The present invention relates to hydraulic transmissions.

A known hydraulic transmission includes an hydraulic coupling having an input driven by a motor and an output shaft coupled to a load, e.g. a boiler feed pump or the drive wheels of a vehicle, the hydraulic coupling catering for reduction of output speed by virtue of slippage occurring in the coupling.

An epicyclic gear is located prior to the hydraulic coupling to provide the desired input speed to the coupling. The coupling slippage occuring at reduced output speed causes considerable inefficiency in the transmission so that there results substantial power losses at reduced speeds. To reduce this power loss, a known improved transmission dispenses with the hydraulic coupling and permits the annulus of the epicyclic gear to rotate for the reduced output speed condition.The power loss due to the annulus slippage is taken from the annulus to drive via gearing a variable geometry hydrostatic pump, for example a swash plate pump, the pressure fluid output from which drives a variable geometry hydrostatic motor (for example a swash plate motor) which in turn has its output mechanically coupled to the input side of the transmission (or to the output) via suitable gearing. Variation of the displacements of the hydrostatic machines can thereby vary the output speed for a fixed input speed. However, these variable geometry hydraulic machines are relatively complex and this can cause a reduction in the reliability of the transmission. It is an object of the present invention to obviate or mitigate this disadvantage.According to the present invention a power transmission includes gearing in the form of an epicyclic gear, a differential or other split power device having a first element movable in a slippage mode for output speed change in the transmission, a hydrostatic pump and a motor set comprising a plurality of hydrostatic motors, one of said pump and motor set being drivingly connected to said first movable element while the other is drivingly coupled to a transmission element of the transmission, conduit means for delivery of pressure fluid from the pump to the hydrostatic motors, and control valve means in said conduit means for selectively controlling the pressure fluid flow to said motors.

Preferably the pump is drivingly coupled to the said first element for driving by slippage movement of the first element.

The gearing preferably comprises epicyclic gear ing, and the pump can be driven from the annulus of the epicyclic gearing while the motors are drivingly coupled to an input or output of the gearing.

By the present invention, the pump and the motors can comprise fixed geometry hydrostatic machines: preferably these machines are screw type machines.

Preferably fluid discharged from the motors is recycled to the pump. Relief valves can be provided to avoid fluid flow of excessive pressure to the motors. Also, gear means are preferably included to enable a reverse mode of operation.

Any element of the epicyclic gear can be selected to act as the slip power element; driver and driven dispositions can be interchanged. The hydrostatic machines acting respectively as a pump engaging the slip element of the epicyclic gear and as motors engaging the output or input element to recover slip power and give reduced output speed can instead operate respectively as a pump engaging the input shaft and motors engaging the slip element of the epicyclic gear to give increased output speed.

In comparison with previous hydraulictransmissions, the transmission of the present invention can dispense with the hydraulic coupling, and there is a considerably increase in efficiency and reduction of damage in the case of seizure when compared with hydraulic couplings and electric slip devices.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a schematic view of a transmission according to the present invention coupling a drive unit to a load: Figure 2 shows a cross sectional side view of the transmission unit of Figure 1.

Figure 3 shows a side view of the fluid control valve of the transmission of Figure 2.

Figure 4 shows an end view of the valve of Figure 3; and Figure 5 shows a schematic illustration of the fluid circuits.

Referring to the drawings, a load 3, such as a boiler feed pump is driven by a drive unit 1, for example an electric motor, steam turbine or diesel engine, via a variable speed transmission 2 which is coupled to the drive unit and load by shafts 4, 5, respectively.

The transmission 2 is in the form of an epicyclic gear and inludes an input 6 coupled to drive shaft4 and a planet carrier 7 integral with input 6, i.e. the trans mission isofstep-uptype. The planet wheel 8 of carrier 7 meshes with sun 9 which is coupled by splines 10 to output 11 which in turn is coupled to driven shaft 5 of load 3.

The planet wheel 8 meshes with annulus 12 which in this case is permitted to orbit freely (or slip).

In the present transmission, three fixed geometry pumps 13 in the form of screw pumps (such as for example MO) (RTM) pumps) are driven by the annulus 12; although only a single pump could be suitable. Further, a plurality (in this example three) of fixed geometry hydrostatic motors 14 of the screw type (e.g. IMO) (RTM) motors) have their output shaft 15 drivingly meshing with a toothed ring 16 on the carrier 7 at the input side of the transmission, and these motors 14 receive pressure fluid from the pumps 13 in the matter shown in Figure 5. Thus (referring to Figure 5) the pumps (or single pump) 13 supply high pressure fluid via line 17 to control valve 18 which in turn selectively directs the pressure fluid to the motors 14 via fluid lines 18.Additionally, a by-pass line 20 can feed back all the pressure fluid to the pumps 13 so by-passing motors 14, and a branch line 21 from line 17 and including relief valve 21A serves to avoid fluid supply at excessive pressure to valve 18 and motors 14, the fluid discharge from motors 14 being recycled to the pumps 13 via line 22.

The master control valve 18 is shown in detail in Figure 3 and is generally of slide valve form wherein axial movement of a selector valve serves to distribute the fluid (oil) to selected ones of the motors 14 orto none. In operation; the control gives eight speeds, namely:synchronous (no flow) i.e. output rotates at max imumspeed; motor one; motortwo: motors one/ two: motor three: motors one/three: motors two/ three: motors one/two/three: by-pass zero speed.

The transverse lines I - VIII and 0 in Figure 3 indicate these selections. At full speed operation, the driven load shaft 5 rotates at input speed times epicyclic step up ratio (i.e. mode I) and the annulus 12 remains stationary: consequently there is no output from the pump 13 and the motors 14 idle. On the other hand, at the non-drive mode with shaft 5 stationary (i.e.

mode 0) maximum slip occurs at the annulus 12 but the by-pass line 20 is open and motors 14 again idle.

Between these extreme modes, various speed modes can be selected by adjusting the control valve to the positions li - VIII so that selected ones of the motors 14 drive input. The control valve is preferably adjusted automatically but manual operation is possible.

Thus in the present embodiment, as an example, three hydrostatic fixed geometry motors can by selecting any of all to be operative or idling, give eight speeds to the output for a fixed input speed.

This large number of suitable speed ratios, or variations of output speed with fixed input speed, eliminates the need for infinitely variable speed. The hydrostatic pump and motor combination transmitting slip power thus acts as a positive gear drive capable of being engaged at full torque without any shock and capable of being instantly disengaged by bypass valves in case of seizure of driven unit thus minimising damage.

Reverse mode of operation can be catered for, for example where the transmission is used in vehicle traction, by the provision of a separate reverse gear train which operates when all the hydrostatic units are operating in the by-pass mode, so that the main drive elements are not disconnected.

The above described transmission dispenses with the need for a fluid torque converter for speed variation. It is estimated that initial costs will be approximately equal for a typical system employing the transmission according to the present invention and for one using the previous transmission having a torque converter, but considerable saving will be provided by the present arrangement during oper aton, particularly where the system is to operate at reduced speeds for considerable periods. Addition ally the use of fixed geometry pumps and hydrostatic motors contributes to a more reliable arrange ment in comparison with the previous transmission utilising variable geometry machines. The present transmission gives a considerable increase in effi ciency and reduction in damage where there is siezure in comparison with hydraulic couplings and electrical slip devices.

Modifications are possible in the system. For example, some other element than the annulus 12 could be selected for use as the slip element, and the driver and driven dispositions can be changed. Thus shaft 11 could be connected to drive shaft 4 while input 6 is connected to load shaft 5. The motors 14 could be drivingly coupled to the slip element (e.g.

annulus 12) while the pumps are driven at the input end (e.g. through carrier 7), to give increased output speed. Instead of an epicyclic gear some other form of split power device could be used, such as for example a differential gear.

Claims (11)

1. A power transmission including gearing in the form of an epicyclic gear, a differential or other split power device having a first element movable in a slippage mode for output speed change in the transmission, a hydrostatic pump and a motor set comprising a plurality of hydrostatic motors, one of said pump and motor set being drivingly connected to said first movable element while the other is drivingly coupled to a transmission element of the transmission, conduit means for delivery of pressure fluid from the pump to the hydrostatic motors, and control valve means in said conduit means for selectively controlling the pressure fluid flow to said motors.

2. A power transmission as claimed in claim 1, wherein the pump is drivingly coupled to the said first element for driving by slippage movement of the first element.

3. A power transmission as claimed in claim 1 or 2, wherein the gearing comprises epicyclic gearing.

4. A power transmission as claimed in claim 3, wherein the pump is driven from the annulus of the epicyclic gearing while the motors are drivingly coupled to an input or output of the gearing.

5. A power transmission as claimed in any one of the preceding claims, wherein the pumps and motors comprise fixed geometry hydrostatic machines.

6. A power transmission as claimed in claim 5, wherein the pumps and motors comprise screw type machines.

7. A power transmission as claimed in any one of the preceding claims wherein fluid discharged from the motors is recycled to the pump.

8. A power transmission as claimed in any one of the preceding claims, wherein relief valves are provided to avoid flow of excessive pressure to the motors.

9. A power transmission as claimed in any one of the preceding claims, wherein gear means are provided to enable a reverse mode of operation.

10. A power transmission as claimed in claim 3, wherein said first element movable in a slippage mode comprises the annules of the epicyclic gear.

11. A power transmission substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.