GB2157460A - Adjustment mechanism for the displacement volume of a displacement machine - Google Patents

Abstract

The adjustment mechanism in accordance with the invention is preferably intended for a radial piston pump (10). Its displacement volume, and consequently its feed current, depends on the position of a piston guide ring, which is movable by means of an adjustable piston (11). Pressure acting on the adjustable piston can be varied by means of a control valve (20). Between the valve body of the control valve and the adjustable piston is fixed a pressure spring (29). The force of this spring is a measure for the actual value of the feed current. A desired force which is formed by a desired value (X1 or X2) works against the force of the spring. The desired value is produced by a producer (23, 24) which has a desired value limiter (40). This can be activated when the feed pressure exceeds an adjustable pressure boundary value. The adjustment of the pressure boundary value occurs by a computer (43) as a function of the quotient from the maximum permissible feed performance (P) and the instantaneous feed current (Q). The latter is measured by a displacement pickup (49) which is connected to the adjustment piston. <IMAGE>

Description

SPECIFICATION Adjustment mechanism for the displace ment volume of a displacement machine The invention relates to an adjusting mecha nism of the kind comprising a machine com ponent determining the displacement volume movable by means of an adjusting piston, which latter can be pressure-loaded; a control valve having a mobile valve body for varying the pressure acting on the adjusting piston; to move the valve body, a theoretical force which corresponds to the respective desired value (X, or X2) of the displacement volume to act on the said valve body against an opposed force (e.g. a spring), which represents the respective actual value of the displacement volume; and a desired value producer for producing the theoretical force which has a desired value limiter, and which can be activated by the feed pressure exceeding a pressure boundary value which can be set (by means of a boundary value producer).

Such and adjusting mechanism is known from Voith Publication G 1 Q58, page 10; it is constructed as a feed current control mecha nism with pressure cut-off. This means the following: As long as the pressure in the transport line does not exceed a permissible boundary value, the feed current can be adjusted to random values by varying a control pressure (deliverable via one of the connections X, or X2). However, if the feed pressure exceeds the respective permissible value, then a so-called pressure cut-off valve (constructed as a pressure limiting valve) is opened thereby for the purpose of the automatic reduction of the control pressure. As a result the feed current is withdrawn to the extent that the feed pressure no longer exceeds the permissible value.In the known appliance the control pressure acts on a control piston which exerts the desired force via a linkage on the valve body of the control valve.

It is obvious that the known mechanism could also be implemented with the same effect with other structural elements. Therefore more general terms are used herein for these structural elements: The control piston producing the desired force is called the "desired value producer". Furthermore the pressure cut-off valve is called "desired value limiter". This desired value limiter has a boundary value producer, with whose help the pressure boundary value which is not to be exceeded can be adjusted. The pressure cutoff valve of the known device has a pressure spring as boundary value producer whose spring tension (= pressure boundary value) is adjustable.

One disadvantage of the known device is that under certain circumstances the displacement machine produces a very high feed current with the maximum feed pressure still permissible. This means that the feed performance (the product of feed current and feed pressure) assumes an extremely high value.

An attempt has therefore been made to provide not only the known pressure cut-off but also a power output limitation device. How ever until now only rather expensive auxiliary equipment has been proposed for this. For example, the power output can be continuously measured, the measured value com pared with a permissible boundary value and the feed current be withdrawn if the boundary value is exceeded. The measurement of the power output can be obtained by the measured values for the feed pressure and for the feed current being multiplied together. However even the required pressure detector is a relatively expensive device.

Therefore the object of the invention is to supplement an adjusting mechanism of the kind discussed above, which is constructed as a feed current control, by a power output limiting device with as little expenditure as possible.

According to this invention adjusting apparatus of the kind discussed above is characterised by the use of the features e) and f) set out in patent claim 1.

The invention primarily makes use of the fact that the feed current of a displacement machine can be measured quite simply by means of a cheap (so-called) displacement pickup, whereby an electrical measured variable is formed. More precisely, this displacement pickup measures the actual value of the displacement volume. With a constant assumed rotational speed this actual value is however also proportional to the actual value of the feed current. In that case use is made of the fact that the maximum permissible feed performance can immediately be presented as an adjustable electrical magnitude, e.g. with the aid of a potentiometer.Consequently according to the invention from the two electrical magnitudes the quotient "Permissible performance/feed current" can be formed with the aid of a simple electric computer which represents a value for the instantaneous permissible feed pressure, the socalled pressure boundary value. Hereby the known formula Performance = Feed current x feed pressure is taken into consideration.

Finally, in accordance with a further step of the invention (characteristic f of the patent claim) the pressure boundary value obtained is supplied to the desired value limiter. In other works: instead of the spring tension which can be varied manually (with the known device), an automatically adjustable pressure boundary value is used in the form of an electrical control magnitude. If the desired value limiter is a pressure limiting valve, as is known, its pressure spring is replaced by a proportional magnet which is controlled by the named electrical control magnitude.

The accompanying drawing schematically shows an embodiment of adjustment mechanism constructed as a feed current control in accordance with the invention.

A displacement pump, which can convey in both directions (preferably a radial piston pump) is marked 1 0. Either the line 1 6 is the pressure line and the line 1 7 the suction pipe or vice versa, depending on the feed direction. The specific displacement volume (feed volume per rotation) can be varied by displacing a correcting element (e.g. a piston guide ring). For this purpose a variable piston 11, against which a smaller opposed piston works, is used, as is already known. The variable piston 11 is acted upon via a line 1 3 and the opposed piston 1 2 is acted upon via a line 14 with pressure medium.The line 1 3 is connected via a control valve 20, line 15, double relief valve 45 and line 15' or 15" to the respective pressure line 1 7 or 1 6. The line 14 is also connected via a double relief valve 44 and line 14' or 14" to the respective pressure line 16 or 17.

The control valve 20 has three positions, as is already known. In the centre position shown the line 1 3 is connected via one pressure-regulating valve respectively both with the line 1 5 and also via a line 1 8 with the pressureless sump 1 9. The pressure working surfaces of the piston 11 and 1 2 and the mentioned pressure-regulating valves are measured so that in the centre position of the control valve 20 equilibrium of forces prevails at the correcting element of the pump 10.If the control valve 20 (in the drawings) is moved to the right the line 1 3 is connected with the line 1 5 and shut off from the sump 1 9. In the reverse case the line 1 3 is connected with the sump 1 9 and cut off from the line 1 5. Both control operations result in alterations to the feed volume of the pump 10 upwards or downwards respectively.

The adjustment of the control valve 20 results with the aid of a lever 21 which can be tilted around a point of rotation 22 (for example arranged approximately in its centre) and is supported with one end at the valve body of the control valve 20, against the compressive force of a spring 29. This is fixed between the adjusting piston 11 and the valve body of the control valve 20, symbolically represented by the mechanical connection 31.

The spring tension is consequently proportional to the instantaneous feed current (with a constant pump rotational speed).

To actuate the lever 21 two pistons 23 and 24 arranged coaxially and abutting one another are used. Between one (23) of the two pistons and the lever 21 is provided a joint-like connection 9 or toothing. The outer front surfaces ("pressure working surfaces" 25 and 26) of the piston can be acted upon in opposite directions by variable control pressures (= desired value). The control pressures are supplied from outside via connections X, and X2 and via lines 27 and 28. The pressure loading of the two pistons 23 and 24 occurs alternately; i.e. the pump 10 conveys when one piston 23 is acted upon in one direction and the other piston 24 is acted upon by pressure in the other direction.

The two control lines 27 and 28 are connected via a double relief valve 38 and a discharge line 39 to a pressure limiting valve 40 (= desired value limiter). Under the conditions described below this can connect the discharge line 39 with the pressureless sump 19. The pressure limiting valve 40 can be moved in the open direction by the feed pressure in the respective pressure line 1 6 or 1 7. This pressure arrives via line 15" or 15' and via double relief valve 45 and lines 1 5 and 15a at the valve body of the pressure limiting valve 40. In the opposite direction an electromagnet 41 which is fed via a line 42 by a variable control current acts as boundary value producer. This is formed in a computer 43.Fixed values for the maximum permissible feed pressure p and for the maximum permissible feed performance P are input to the computer 43, symbolically represented by lines 46 and 47. Furthermore, via the line 48 is input an electrical magnitude proportional to the respective feed current Q. This is formed by a displacement pickup 49 which (symbolically shown) is actuated via the mechanical connection 31 by the adjustable piston 11.

As already explained above, the computer 43 forms the quotients from values P and Q and supplies these via the line 42 as the socalled pressure boundary value to the pressure limiting valve 40.

If the feed pressure supplied via the line 15a exceeds the above-named pressure boundary value, then the pressure limiting valve 40 opens, whereby the control pressure supplied respectively from outside (in line 27 or 28) is reduced. Hereby the feed performance is limited to the permissible boundary value P respectively used.

The control described acts correspondingly if the instantaneous feed pressure (in line 15a) exceeds the maximum permissible boundary value p input via the line 46 into the computer 43. The computer 43 constantly compares the quotient P/Q with the above-named value p. The respective greater value is supplied via the line 42 to the pressure limiting valve 40.

In the drawings it is assumed that for both feed directions of the pump 10 the same maximum permissible boundary values (p,P) apply for the feed pressure and feed performance. However deviating from this different boundary values can also be input for the two feed directions.

The control of the feed current with power output limitation in accordance with the invention has been explained with an example in which the feed current desired value producer is constructed as pistons 23, 24 which can be hydraulically acted upon. Instead similar pistons which can be actuated pneumatically could also be provided.

Finally the desired value limitation in accordance with the invention can even be used if two electromagnets with variable magnetic force are provided instead of the pistons. In this case the desired value limiter (40) is not constructed as a pressure limiting value but as a current limiter which limits the supply currents for the electromagnets. Moreover to line 1 5 instead of line 15a is connected a pressure transmitter which supplies an electrical magnitude for the feed pressure to the current limiter. This compares the above-named electrical magnitude with the value present in line 42, which represents the quotient P/Q, as explained,

Claims (3)

1. An adjustment mechanism for adjusting the displacement volume of a displacement machine, such as a radial piston pump (10) of the kind with the following features: a) a machine component determining the displacement volume movable by means of an adjusting piston (11), which letter can be pressure-loaded; b) a control valve (20), having a mobile valve body for varying the pressure acting on the adjusting piston (11) (line 13), c) to move the valve body (20) a theoretical force which corresponds to the respective desired value (X1 or X2) of the displacement volume to act on the said valve body against an opposed force (e.g. a spring 29), which represents the respective actual value of the displacement volume; and d) a desired value producer (23,24) for producing the theoretical force which has a desired value limiter (40), and which can be activated by the feed pressure (line 15a) exceeding a pressure boundary value which can be set (by means of a boundary producer 41); characterised by the following features: e) a per se known electronic computer (43) forms the quotient from the actual value (Q) of the feed current (= product from the actual values of the displacement volume and the rotational speed) and from the maximum permissible feed performance (P); f) the computer (43) is connected via a signal line (42) with the boundary value producer (41) of the desired value limiter (40) so that the value of the named quotient can be supplied as pressure boundary value to this.

2. An adjustment mechanism according to Claim 1, characterised in that the desired value limiter (40) is constructed as a pressure limiting valve, on whose valve body the feed pressure (line 15a) acts in the open direction, and in that an electromagnet (41) works in the closed direction, the magnetic force of which can be controlled in proportion to the named quotient.

3. An adjustment mechanism arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawing.