GB1599427A

GB1599427A - Power assisted steering mechanisms

Info

Publication number: GB1599427A
Application number: GB7164/77A
Authority: GB
Original assignee: Adwest Engineering Ltd
Current assignee: Adwest Engineering Ltd
Priority date: 1977-11-12
Filing date: 1977-11-12
Publication date: 1981-09-30
Also published as: DE2844146A1; FR2408502A1; DE2844146C2; ES474983A1; FR2408502B1; SE7808303L

Description

(54) POWER ASSISTED STEERING MECHANISMS (71) We, ADWEST ENGINEERING LIMITED, of The Aerodrome, Woodley, Reading, Berkshire, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to power assisted rack-and-pinion steering mechanisms for motor vehicles.

A known kind of power assisted rack-andpinion steering mechanism for motor vehicles comprises a toothed rack at least one end of which is connectable with the steerable road wheels of a motor vehicle and which is movable longitudinally to impart steering to said road wheels, a pinion in meshing engagement with the rack which is connectable with the steering control of a motor vehicle and which is rotatable to impart longitudinal movement to the rack, a cylinder, a double acting piston slidable in the cylinder and connected to the rack for movement therewith, and a control valve associated with the pinion for directing fluid under pressure to one side or the other of the double acting piston to provide power assistance (such a steering mechanism being hereinafter and in the claims hereof referred to as "a power assisted steering mechanism of the kind described"). With a power assisted steering mechanism of the kind described the fluid under pressure is generally supplied by a pump which is driven by the engine of the motor vehicle. A difficulty which is encountered with power assisted steering mechanisms of the kind described is that of pressure build-up when the piston is at the end of its stroke in either direction. Whilst no harm accrues to the steering mechanism itself under such a condition, the build-up of pressure can have a detrimental effect on the pump and can even lead to pump failure if the said condition maintains for more than a few seconds at a time.

In our co-pending Patent Application Serial No. 1,484,942 there is described and claimed steering mechanism of the kind described, comprising a cylinder port at each end of the cylinder, the cylinder ports being connectable through said control valve one with a source of fluid under pressure and the other with a fluid reservoir, and a pressure relief port associated with each said cylinder port, each pressure relief port communicating with its associated cylinder port and being so spaced therefrom axially of the cylinder that when the piston is at the end of its travel in either direction the pressure relief port at that end of the cylinder will be on the opposite side of the piston to its associated cylinder port so that fluid under pressure on said opposite side of the piston can flow to the associated cylinder port. Thus, whenever the piston is at the end of its travel in either direction, the pressure relief port at that end of the cylinder communicates the cylinder on the high pressure side of the piston with its associated cylinder port on the low pressure side of the piston and so reduces the risk of damage to the pump supplying the fluid under pressure.

The present invention aims to provide an improvement in or modification of the invention claimed in our said co-pending Patent Application Serial Number 1,484,942.

The present invention provides a power assisted steering mechanism of the kind described, wherein the cylinder comprises passage means whereby communication can be provided between the cylinder on one side of the piston with the cylinder on the other side of the piston when the piston is substantially at the end of its travel in either direction, said passage means comprising one or more grooves or channels formed in the internal wall of the cylinder.

Thus it will be seen that whenever the piston is at the end of its travel in either direction, the pressure relief passage means at that end of the cylinder will communicate the cylinder on the high pressure side of the piston with its associated cylinder port on the low pressure side of the piston and so prevent damage to the pump supplying the fluid under pressure.

The pressure relief passage means is in open communication with its associated cylinder port, no valves or the like being provided therebetween.

When the piston is at the full extent of its travel towards one end of the cylinder, and fluid under pressure is supplied to the cylinder port at that end of the cylinder to move the piston back in the opposite direction, the fluid under pressure will also pass by way of the pressure relief passage means to the cylinder on the opposite side of the piston to the cylinder port. To return the piston past the pressure relief passage means the invention relies upon the natural "wind-up" of the tyres of a vehicle and the geometry of the steering linkages, both of which will tend to return the steering to a central position. However, to assist in returning the piston over the pressure relief passage means the pressure relief passage or passages preferably provide(s) restricted passage for the fluid under pressure as compared with the cylinder port itself so that a higher pressure will prevail on the cylinder port side of the piston.

The number of grooves or channels formed in the internal wall of the cylinder is selected according to the facility with which they can be machined or otherwise formed within the cylinder. Additionally the number of grooves or channels, the size of each and the spacing and position of adjacent grooves or channels is also determined by the degree of control required in regulating the flow of pressurized hydraulic fluid through them and by the necessity of ensuring that the provision of the grooves or channels does not have an adverse effect in causing excessive wcar cn the peripheral surface of the piston as the latter moves across them. In the latter aspect, it is important though not critical that each individual groove or channel should be as narrow as possible commensurate with allowing controlled but free flow of fluid therethrough.

In a preferred embodiment of the invention, the groves or channels are spaced axially of one another in a manner such as to provide for a gradual fall in pressure as the piston approaches the end of its stroke, i.e. at the full lock position of the steering system. This provision of gradual control of the fall in pressure is especially important if, when the steering mechanism is installed in a vehicle, the pressure in the cylinder approaches the relief valve pressure setting of the pump (which can occur when the steering is actuated whilst the brakes are applied). If, for example, the pump pressure setting is 1000 p.s.i. and the operating pressure is 900 p.s.i. whilst the grooves or channels determine a 200 p.s.i. restriction, then the pressure drop would result in a lowpressure-side pressure of 700 p.s.i.

The result of this would be that at the steering wheel the effort required to tum the wheel would increase from a light effort of say 4 to Slbs up to say 40 to 501bs, i.e. a tenfold increase in required effort. On the other hand, if the grooves or channels are displaced axially relative to one another, each gradually increases the total effective cross-sectional area thereby causing a gradual increase in the effort required by a driver, instead of a sudden increase. As far as the driver is concerned this provides a tactile feedback to the driver who would not need to increase his effort.

By having a pressure cut-off facility, extra strain generated in the steering mountings can be avoided since no manual effort will be added to the maximum possible power effort. It can only be power or manual effort at the full lock position. This does not exclude the possibility of applying manual effort as well as power effort at some position other than full lock and hence it is necessary to provide a pressure relief valve. This condition could only be achieved under extreme conditions, but design considerations must allow for the possibility.

In a further embodiment of the present invention, it is not necessary that the grooves or channels be longer than the axial length of the piston, provided that the piston is itself provided with lead-in grooves or the like arranged to co-operate with the grooves or the channels in the cylinder.

The invention will be further particularly described with reference to the accompanying drawing which is a sectional elevation of a power-assisted rack-and-pinion steering mechanism according to the invention.

The rack-and-pinion power assisted steering mechanism illustrated comprises a toothed rack 1 slidably mounted within a housing 2 and a pinion 3 in meshing engagement with the rack 1. The rack 1 has a piston rod 4 formed integrally therewith on which is fixedly mounted a piston 5 slidable within a cylinder 6 defined by the housing 2. The opposite ends of the toothed rack 1 and integral piston rod 4 are connectable to the steerable road wheels of a motor vehicle by way of ball joints 7 and steering linkages 8. Each ball joint 7 is enclosed within a flexible bellows member 9. The pinion 3 is connectable by way of a rotary control valve 10 of conventional kind with the steering control of a motor vehicle. The rotary control valve 10 serves to direct fluid under pressure from a source thereof (not shown) to the interior of the cylinder 6 by way of one of two cylinder ports (not shown) and to connect the other of said cylinder ports to reservoir. Thus, for example, when the pinion 3 is rotated by actuation of the steering control of a motor vehicle to which the steering mechanism is fitted to move the rack 1 towards the left in the aspect of the drawing, the rotary control valve 10 will operate to direct fluid under pressure to the cylinder port at the right hand side of the drawing and to connect the cylinder port at the left hand side of the drawing to reservoir so that the fluid under pressure will act on the piston 5 on the right hand side thereof to provide power assistance in moving the rack 1 towards the left. Associated with the right hand port is a plurality of radially spaced pressure relief passages provided by grooves 11 formed in the interior wall of the cylinder 6 which provide communication with the cylinder port.

Associated with the left hand port is a plurality of radially spaced pressure relief passages provided by grooves 12 formed in the interior wall of the cylinder 6 which provide communication between the left hand port by way of an annular passage 13, formed between the cylinder 6 and a rigidly mounted internal sleeve 14, and the passages or grooves 12. The pressure relief passages 11 and 12 are so spaced axially of the cylinder from their associated ports respectively that when the piston 5 is at the extent of its travel at one end or the other of the cylinder 6, the pressure relief passages at that end of the cylinder will provide communication between opposite sides of the piston 5, thus allowing fluid under pressure to pass from the high pressure side of the piston 5 by way of the pressure relief passages into the associated port and so to reservoir to relieve the pressure on the high pressure side of the piston and prevent damage to the pump supplying the fluid under pressure. The piston 5 is returned across the pressure relief passages by the natural "wind-up" of the tyres of a motor vehicle to which the steering mechanism is fitted and the inherent geometry of the steering linkages which tend to bias the steerable road wheels of the vehicle towards an on-centre position and by the fact that the pressure relief passages 11 and 12 provide only a restricted flow for the fluid under pressure so that fluid under pressure supplied to the port at that end of the cylinder will create a higher pressure on the cylinder port side of the piston than on the pressure relief side of the piston to return the piston back past the pressure relief passages.

The pressure relief passages 11 and 12 will generally be provided in addition to the pressure relief valve normally provided in the pressure fluid circuit since it is at the normal relief valve pressure that damage can occur to the pump providing the fluid under pressure.

In a preferred embodiment of the invention, the relief passages or grooves 11 and 12 at each end of the cylinder are spaced axially relative to adjacent passages or grooves in order to control the flow of pressure fluid therethrough as the piston reaches the end of its stroke.

WHAT WE CLAIM IS: 1. A power assisted steering mechanism of the kind described, wherein the cylinder comprises passage means whereby communication can be provided between the cylinder on one side of the piston with the cylinder on the other side of the piston when the piston is substantially at the end of its travel in either direction, said passage means comprising one or more grooves or channels formed in the internal wall of the cylinder.

2. A power-assisted steering mechanism according to claim 1 wherein the passage means comprises a plurality of grooves or channels spaced axially of onc another.

3. A power-assisted steering mechanism according to either one of claims 1 or 2 wherein the piston is provided with one or more lead-in grooves or channels arranged to co-operate with the one or more grooves or channels formed in the internal wall of the cylinder.

4. A power-assisted steering mechanism substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. grooves 11 formed in the interior wall of the cylinder 6 which provide communication with the cylinder port. Associated with the left hand port is a plurality of radially spaced pressure relief passages provided by grooves 12 formed in the interior wall of the cylinder 6 which provide communication between the left hand port by way of an annular passage 13, formed between the cylinder 6 and a rigidly mounted internal sleeve 14, and the passages or grooves 12. The pressure relief passages 11 and 12 are so spaced axially of the cylinder from their associated ports respectively that when the piston 5 is at the extent of its travel at one end or the other of the cylinder 6, the pressure relief passages at that end of the cylinder will provide communication between opposite sides of the piston 5, thus allowing fluid under pressure to pass from the high pressure side of the piston 5 by way of the pressure relief passages into the associated port and so to reservoir to relieve the pressure on the high pressure side of the piston and prevent damage to the pump supplying the fluid under pressure. The piston 5 is returned across the pressure relief passages by the natural "wind-up" of the tyres of a motor vehicle to which the steering mechanism is fitted and the inherent geometry of the steering linkages which tend to bias the steerable road wheels of the vehicle towards an on-centre position and by the fact that the pressure relief passages 11 and 12 provide only a restricted flow for the fluid under pressure so that fluid under pressure supplied to the port at that end of the cylinder will create a higher pressure on the cylinder port side of the piston than on the pressure relief side of the piston to return the piston back past the pressure relief passages. The pressure relief passages 11 and 12 will generally be provided in addition to the pressure relief valve normally provided in the pressure fluid circuit since it is at the normal relief valve pressure that damage can occur to the pump providing the fluid under pressure. In a preferred embodiment of the invention, the relief passages or grooves 11 and 12 at each end of the cylinder are spaced axially relative to adjacent passages or grooves in order to control the flow of pressure fluid therethrough as the piston reaches the end of its stroke. WHAT WE CLAIM IS:

1. A power assisted steering mechanism of the kind described, wherein the cylinder comprises passage means whereby communication can be provided between the cylinder on one side of the piston with the cylinder on the other side of the piston when the piston is substantially at the end of its travel in either direction, said passage means comprising one or more grooves or channels formed in the internal wall of the cylinder.