EP0045928B1

EP0045928B1 - Power transmission

Info

Publication number: EP0045928B1
Application number: EP19810106075
Authority: EP
Inventors: Albin Joseph Niemiec
Original assignee: Vickers Inc
Current assignee: Vickers Inc
Priority date: 1980-08-11
Filing date: 1981-08-04
Publication date: 1985-11-21
Also published as: JPS5752686A; IN153744B; DE3172983D1; EP0045928A1; CA1160538A; JPH0138708B2; AU545989B2; AU7185081A

Description

The invention relates to a power transmission as set forth in the preamble of claim 1. Hydraulic pumps for assisted steering are of the fixed displacement type and are driven at variable speeds by the engines of the automotive vehicles. In order to get predetermined constant flow or volume, a flow control valve is provided for bypassing or returning that portion of the pump fluid surpassing the predetermined constant volume of flow.
Power transmissions of the kind as described in the precharacterizing portion of claim 1 are already known (US-A-2,880,674), wherein an accelerated fluid jet is used to draw fluid from an assigned-to tank passage and creates pressure in a pertinent inlet to the connected-to pumping region of the pump in order to save energy and to better fill the pump at higher speeds. In the event of the two pumping regions (as is usual with pumps for assisted steering) two 'first' tank passages and two inlets are connected to the valve bore of the flow control valve, yet no fluid is flowing back into the valve bore so that there is no communication between the not-assigned-to 'first' tank passage and the not-pertinent inlet. For the positive effects referred to above, a restricted size of the first passage is a precondition. In such a system, at lower speeds and in a cold environment, the flow through the inlet of restricted size is such that excessive inlet vacuum and therefore cavitational damage can occur, especially when the fluid is very viscous, and this danger is also present at larger control flows.
Therefore, it is an object of the invention to create a power transmission system of the kind referred to wherein the inlet vacuum when starting the pump in a cold environment is not so excessive or is even avoided and replenishing the flow at the higher speeds is further improved.
A power transmission is known (GB-A-2,019,940) wherein a tank passage is brought into communication with a passage that communicates with a venturi throat. The creation of a strong jet to aspirate fluid is not described and, due to the inlet configuration shown, probably not possible.
The object referred to above is fulfilled with the teaching of claim 1.
With the power transmission system of the invention, replenishing is enhanced both at low speeds when the fluid can be very viscous, and at higher speeds when larger control flows may also give rise to cavitational damages. In both cases the inlet pressure to the pumping region is increased. Further embodiments of the invention are disclosed in the dependent claims.
The single drawing is a partly schematic view of a hydraulic system embodying the invention.
The hydraulic system comprises a fixed displacement pump 10 driven by a prime mover such as the engine of an automotive vehicle to supply fluid through an orifice 11 to a steering booster 12 such as a linear or rotary hydraulic motor. In most operating systems, the return of fluid through a line 13 is to a reservoir 14. Fluid from the reservoir flows through a first tank line 15 and a passage 16 to the inlet 17 of the pump. A flow control valve 18 comprising a spool 19 having spaced lands 20, 21 is provided in a bore 22 and is operable in accordance with well known practice to bypass the fluid when the volume exceeds a predetermined rate. A line 23 extends from the outlet of the pump and has an orifice 24 therein to the spring end of the spool. When the volume exceeds a predetermined rate, the land 21 moves to the broken line position causing a portion of the fluid to flow through line 25 between the land 21 and the edge of the bore into the inlet 17. The passage 16 aspirates further hydraulic fluid flow through the inlet thereby supplementing the flow of fluid to the pump at the higher speed. Such systems are old and shown in the aforementioned United States Patents 2,880,674 and 3,207,077 which should be referred to for details of construction.
In accordance with the invention, an auxiliary passage 26 is provided and communicates as shown diagrammatically by a line 27 with the reservoir 14. Passage 26 functions to provide supplemental flow of hydraulic fluid to the inlet 17 at the lower speeds. It is noted that the passage 26 is located upstream from the passage 16 and is hydraulically isolated from the fluid applied to the flow control valve when the flow control valve is in closed position.
Thus, a hydraulic system embodying the invention provides improved filling at the inlet at the lower speeds before the flow control valve begins to bypass the discharge volume, lowers the sound level, reduces the wear and maintenance that might be caused by vacuum and cavitational damage. The additional passage also functions to increase the pressure at the inlet supplementing the supercharging normally provided by the flow control valve.

Claims

1. A power transmission comprising a fixed displacement pump (10) driven by a prime mover operable at variable speeds in a hydraulic system, where it is desired to have a predetermined volumetric flow to a load (12), said pump (10) including at least a pumping region having an inlet (17) and an outlet, a first tank line (15) including at least a first passage (16) to the inlet (17) of the pumping region and a flow control valve (18) including a valve bore (22), a spool (19) having first (20) and second (21) lands, the said second land (21) operable to return a portion of the hydraulic fluid from the outlet of the pump (10) through a return line (25) to the inlet (17) of said pumping region, the said second land (21) moving to a position where the return flow through the return line (25) springs between the second land (21) and the valve bore (22) into the inlet (17) and aspirates hydraulic fluid from the first tank passage (16) when the pump (10) is operated at higher speeds, characterized in that

upstream of the first tank passage (16), a second tank line (27) is provided having a second, auxiliary, passage (26) which extends through the flow control valve (18) between the two lands (20, 21) and, therethrough, is at all times in communication with the inlet (17).

2. The power transmission as set forth in claim 1, characterized in that the flow control valve (18), the return line (25), the first passage (16) and the second passage (26) are in the same housing as the pump (10).

3. The pump transmission as set forth in claim 1 or 2, characterized in that only the return line (25) and the auxiliary passage (26) are intersecting the valve bore (22) in similar flowing directions and upstream of the first tank line (15) so as to join to the first passage (16) in the inlet (17), when the second land (21) is in its position permitting return flow.