GB2057371A - Hydrostatic steering system with torsion rod - Google Patents

Abstract

A hydrostatic steering system comprises a metering pump 2 and an axially slidable valve member 4. A cardan shaft 12, located in a bore 6 of the valve member 4, is connected pivotably and rotationally fast at one end to the rotor 14 of the pump 2, and at the other end to the valve member 4 via meshing teeth 16; 18. A torsion rod 34 is located in axial blind bores 26 and 28 respectively of the input shaft 8 and the cardan shaft 12. The bore 28 extends at least as far as the vicinity of the output end of the cardan shaft 12. One end of the torsion rod 34 is pivotably connected by way of a transverse pin 30 to the cardan shaft 12 adjacent the inner end of the bore 28. The other end of the torsion rod 34 is pivotably connected to the input shaft 8 by way of a transverse pin 32 adjacent the inner end of the bore 26. The torsion rod 34 engages the pins 30 and 32 respectively by means of its bifurcated ends 36 and 38. The construction according to the invention is intended to minimise the possibility of jamming of the valve member 4. <IMAGE>

Description

SPECIFICATION Hydrostatic steering system with torsion rod This invention relates to a hydrostatic steering system of the kind comprising a hand and metering pump, a control valve controlling said pump and having a slidable valve member and a cardan shaft which is connected, pivotably and rotationally fast, on the one hand to the rotor of said pump and on the other hand to the valve member and to one end of a torsion rod which is located in axis-parallel bores in the cardan shaft and in an input shaft and the other end of which is connected rotationally fast to the input shaft, the torsion rod when twisted acting via a cam or the like to displace the valve member axially.

Such steering system is disclosed in a Company leaflet of Zahnradfabrik Friedrichshafen AG, Schwä- bisch Gmund Works, published in December 1975 under the title "ZF-Ross Servostat, hydrostatic steering system, Series 8450-8455".

Hydrostatic steering systems of the foregoing kind can be used with relatively slowly running vehicles and are advantageous in cases where it is difficult to provide a mechanical connection between the steering gear and the steered wheels, for example, in the case of self-propelled building and utility vehicles, fork-lift trucks, tractors and harvester threshers.

Hydrostatic steering systems of the foregoing kind can also be used for steering ships.

The operation of a steering system of the foregoing kind is known from the afore-mentioned Company leaflet. In normal operation of the system the pump acts as a metering pump for the pressurised liquid delivered by a high-pressure pump and operates as a hand pump in the event of failure of the high-pressure pump. It is therefore called a "hand and metering pump".

To obtain smooth steering, the valve member of the control valve must be displaceably mounted in the steering gear casing with as little friction as possible. To this end it is necessary to avoid radial forces which urge the valve member against the casing, since if radial forces occur, the gap between the valve member and the steering gear casing is reduced in size. Then there is a risk that due to the differential pressures in the various control ducts the valve member will be even further forced against the steering gear casing. In that case the working fluid may be forced away and the valve member may contact the steering gear casing directly, resulting in a high degree of friction.If the valve member is to be moved by actuation of the steering wheel out of its inoperative position, undersirable jamming occurs at the steering wheel, so that during the transition from adhesive to sliding friction at first a relatively high force must be exerted which is then suddenly reduced to a lower value, and this results in unsmooth steering and a poor road feeling.

In the known steering system undesirable radial forces are exerted on the valve member for two reasons.

One torsion rod end, constructed as a cylindrical bolt, engages in the end of a cylindrical bore in the input shaft - i.e., the steering spindle - where it is pinned. Due to machining inaccuracies, the necessary clearance and the pinning operation, as a rule the axis of the torsion rod extends at a small angle to the axis of the input shaft. During mounting the torsion rod is bent into its correct position. It then exerts a radial force on the valve member, since in the known steering system its other end is connected to the cardan shaft end adjacent the input shaft - i.e., such end acts on the cardan shaft at the same place at which the cardan shaft engages via teeth with the valve member.

Quite independently of this, further radial forces are exerted on the valve member by the axis of the input shaft always extending at a considerable angle to the axis of the cardan shaft, since of course the output end of the cardan shaft must rotate eccentrically with the rotor of the metering pump. Consequently the torque vectors of the input shaft and the cardan shaft enclose an angle with one another, and this means that a supporting torque inevitably occurs. This will be explained in greater detail with reference to Figure 5 of the accompanying drawings.

The effect of the supporting torque is to tilt the valve member around an axis lying transversely of its longitudinal direction - i.e., to force its ends in opposite directions radially against the bore in the steering gear casing.

It is an object of the invention to provide a hydrostatic steering system in which no radial forces are exerted by the valve member of the control valve on the steering gear casing in the inoperative position, and at most negligible radial forces are so exerted when drive torques are exerted.

Accordingly, the present invention consists in a hydrostatic steering system comprising a hand and metering pump, a control valve controlling said pump and having a slidable valve member and a cardan shaft which is connected, pivotably and rotationally fast, on the one hand to the rotor of said pump and on the other hand to the valve member and to one end of a torsion rod which is located in axis-parallel bores in the cardan shaft and in an input shaft and the other end of which is connected rotationally fast to the input shaft, the torsion rod when twisted acting via a cam or the like to displace the valve member axially, characterised in that the torsion rod is also pivotably connected to the input shaft, and the bore formed in the cardan shaft extends at least as far as the vicinity of the output end of the cardan shaft.

Since the torsion rod is also pivotably connected to the input shaft, the rod acts as a two-joint rod which cannot transmit any radial forces.

Since the rod extends as far as possible into the bore in the cardan shaft it is only there connected pivotably to such shaft, the afore-mentioned supporting torque occurs at a place where it is to the greatest part absorbed by the rotor of the metering pump and only to a negligible extent by the valve member, so that the effect of the supporting torque is in practice rendered harmless.

In practice, therefore, no radial forces are any longer exerted by the valve member on the steering gear casing, and the afore-mentioned jamming is obviated.

Advantageously, the axial bore in the cardan shaft extends as far as its output end in the rotor of the pump, where the torsion rod is articulated. In this case the reaction torque occurs exclusively in the rotor, where is is harmless, and the valve member is completely relieved of the reaction torque and the resulting radial forces.

Assembly is simplified by a further optional feature of the invention, whereby entraining pins can be inserted in the drive shaft and cardan shaft, while all that is required subsequently is to insert the torsion rod by its bifurcated ends.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example an embodiment thereof, and in which Figure 1 is a longitudinal section through a steering system according to the invention, Figure 2 is a cross-section on the line ll-ll of Figure 1, Figure 3 is a cross-section on the line Ill-Ill of Figure 1, Figure 4 is a plan view of a portion of the valve member of the control valve, in the direction of the arrow IV in Figure land Figure 5 shows vectors explaining the reaction torque on actuation of the steering system.

Figure 1 shows a control valve 1 and a hand and metering pump 2 rigidly screwed to the control valve, which has in known manner a tubular valve member 4 in whose interior 6 a prolongation 8 of a steering spindle 10 and a cardan shaft 12 engage.

The cardan shaft 12 connects the valve member 4 rotationally fast to a rotor 14 of the pump 2 and enables the rotor 14 to rotate eccentrically in relation to the axis of valve member 4. As shown in Figure 2 and 3, the valve member 4 has inner teeth 16 in which teeth 18 of the cardan shaft 12 engage rotationally fast. In contrast, the prolongation 8 of the steering spindle 10 engages via cams 20 in wide tooth gaps 19 ofthe inner teeth 16 ofvalve member 4, so that the prolongation 8 can rotate to a limited extent in relation to the valve member 4.

As shown in Figure 4, the valve member 4 is provided with an oblique slot 22 which acts as a guide for a pin 24 secured to the prolongation 8 and engaging the slot 22 without clearance.

The prolongation 8 has a coaxial blind bore 26 and the cardan shaft 12 has a coaxial blind bore 28.

Entraining pins 30 and 32 are respectively secured in the cardan shaft 12 and steering spindle 10 at the inner ends of each of the bores. A torsion rod 34 has bifurcated ends 36 and 38 via which it engages without clearance the entraining pins 30 and 32 respectively.

When the steering spindle 10 is rotated out of its central position against the resistance of the steered wheels, which resistance acts on the rotor 14, the torsion rod 34 is twisted and the valve member 4 axially displaced via the entraining pin 24 and the oblique slot 22. The cams 20 of the prolongation 8 co-operate with the innerteeth 16 of the valve member 4 to form abutments to conserve the torsion rod. In contrast, the valve member 4 is connected rotationally fast to the rotor 14 via the meshing teeth 16 and 18 of the valve member 4 and cardan shaft 12.

The pivotable arrangement of the torsion rod 34 in both directions prevents it from getting bent during mounting and exerting radial forces on the valve member 4.

With a steering movement against a resistance torque of the rotor 14 of the pump 2, an input torque is initiated in the steering spindle 10, while the output torque in the cardan shaft 12 coincides with its axis.

Figure 5 shows the torques by means of vectors, the angle between the input and output torques being exaggerated. The input torque is represented by the line a, the counteracting output torque being represented by the line b. This can be made up by vectorial addition of a reaction torque c, extending in the direction of the input torque and having the same value as the latter, and a supporting torque d which extends at right angles thereto and at right angles to the input torque a. The effect of the supporting torque is to tilt, by vector d, that part which is acted on directly by the input torque.If then, as in the known hydrostatic steering system, the steering spindle 10 is connected via the torsion rod to the adjacent end of the cardan shaft (for comparison: the right-hand end, as viewed in Figure 1), the supporting torque acts via such end of the cardan shaft directly and practically to the full extent on the valve member 4. At this place, therefore, a strong radial force acts on the valve member 4 transversely of the plane of the drawing.

The invention makes the arrangement substantially more advantageous, since the torsion rod 34 engages not the right-hand end of the cardan shaft 12 (Figure 1), but substantially its left-hand end. The supporting torque d exerted on the cardan shaft tends to tilt the cardan shaft around an axis in the plane of the drawing. The supporting torque acts by radial forces transversely of the plane of the drawing on one hand on the rotor, and on the other hand on the valve member 4. Since the supporting torque is initiated adjacent the left-hand end (as viewed in Figure 1) of the cardan shaft, in accordance with the law of levers the radial force acting on the valve member is only about one-fifth of that acting on the rotor. However, the radial force acting on the rotor is harmless.

The arrangement can be further improved by taking the coaxial bore 28 as far as the cardan shaft left-hand end, which meshes with the rotor 14 so that the radial force still exerted by the cardan shaft on the valve member becomes negligible.

Claims (4)

1. A hydrostatic steering system comprising a hand and metering pump, a control valve controlling said pump and having a slidable valve member and a cardan shaft which is connected, pivotably and rotationally fast, on the one hand to the rotor of said pmp and on the other hand to the valve member and to one end of a torsion rod which is located in axis-parallel bores in the cardan shaft and in an input shaft and the other end of which is connected rotationally fast to the input shaft, the torsion rod when twisted acting via a cam or the like to displace the valve member axially, characterised in that the torsion rod is also pivotably connected to the input shaft and the bore formed in the cardan shaft extends at least as far as the vicinity of the output end of the cardan shaft.

2. A steering system according to claim 1, wherein the bore in the cardan shaft extends as far as the cardan shaft output end meshing with the rotor of said pump, and the output end of the torsion rod is pivotably connected to the cardan shaft only inside such output end.

3. A steering system according to claim 1 or 2, wherein an entraining pin extends transversely through the ends of the bores in the input shaft and cardan shaft, which ends face away from one another, and the torsion rod has bifurcated ends which engage around the entraining pins.

4. A hydrostatic steering system for vehicles, substantially as herein described with reference to and as shown in the accompanying drawings.