GB2112138A - Rotational displacement measuring instrument - Google Patents

Abstract

An instrument in which a rotational displacement of a measuring element 3 about a shaft 2 is amplified and transmitted by a sector gear 4 and a gear train 8 to the pointer 6 of an indicating device 5 has means for adjusting the zero position of the pointer. The sector gear 4 is mounted on the shaft 2 for rotation with the measuring element 3 and is biassed to a substantially central neutral or rest position by a pair of springs 17 and 18 which act on the sector gear in opposition to each other. One of the springs 17 is stronger than the other, and the instrument is provided with fine adjusting means 19 which engages the stronger spring 17 to limit the extent to which it can push the sector gear 4 against the weaker spring 18. The means 19 therefore determines the rest position of the sector gear 4 and the measuring element 3, and the means 19 is adjusted until the pointer registers zero. <IMAGE>

Description

SPECIFICATION Rotational displacement measuring instrument This invention relates to instruments in which a rotational displacement of a measuring element is amplified and transmitted to a measurement indicator by a mechanism which includes a sector gear mounted on the body of the instrument so that it moves in response to rotational displacement of the measuring element.

In known instruments of this kind the mechanism for amplifying and transmitting rotational displacement of the measuring element to the indicator generally comprises the sector gear and a gear train operationally associated with the sector gear, and the indicator comprises a pointer which is driven by the gear train and which indicates the displacement on a dial. Usually the pointer is arranged to indicate zero displacement when the measuring element is at a substantially central position so that it is able to measure substantially equal rotational displacements in opposite directions.In order to bias the measuring element to its central zero position it has been proposed to provide a pair of springs to act on the sector gear in opposition to each other, the springs being of substantially equal strength so that the sector gear has an equilibrium rest position at the centre of its range of movement and in which it holds the pointer and the measuring element in their zero positions.

The springs may be tension springs which puil on the sector gear from opposite sides, or leaf springs which push on opposite sides of the sector gear. In either case, however, even though the two springs are of nominally equal strength, they tend to have variations in resiliency and the forces they exert on the sector gear may vary depending on the way the instrument is assembled, with the result that difficulties are encountered in achieving an accurate and reliable zero position.

The aim of the present invention is to overcome this difficulty by providing the instrument with means for adjusting the zero position of the measuring element so that it can be adjusted to correspond with the zero position of the indicator.

To this end, according to the invention, an instrument of the kind described is characterised in that the measuring element is biassed to a substantially central zero position by means of a pair of springs which act on the sector gear in opposition to each other, and in that the two springs are of different strengths and the body carries a finely adjustable stop for engaging the stronger of the two springs to limit the extent to which the sector gear can be moved by the stronger spring against the action of the weaker spring. The adjustable stop hence determines the zero position of the measuring element since the springs will always return the sector gear to the position determined by the stop, and the stop is therefore adjusted until the indicator registers zero.

An example of an instrument in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a vertical section through the instrument; Figure 2 is a section through a part of the instrument and taken along the line Il-Il in Figure 1 ; and, Figure 3 is also a section through a part of the instrument, but taken along the line Ill-Ill in Figure 1 and drawn to a larger scale.

Referring to Figure 1, the instrument comprises a body 1 formed as a one-piece box-like construction by aluminum die casting or some other similar process. A shaft 2 is secured to one end portion of the body 1, and one end of a measuring element 3 (the upper end in the drawing) is rotatably mounted on the shaft 2. Also rotatably mounted on the shaft 2 is one end (the lower end in the drawing) of a sector gear 4. The measuring element 3 and the sector gear 4 are pressed into contact with each other by a spring washer (not shown) so that the sector gear 4 and the measuring element 3 will rotate together on the shaft due to the frictional force between them.

Attached to the body 1 is an indicating device 5 which functions to indicate measurement of rotational displacement of the measuring element 3 about the shaft 2. The device 5 comprises a pointer 6 fixed to one end of a shaft 6a which is rotatably mounted relative to the body 1, and a fixed dial plate 7. The pointer 6 is operationally connected to the measuring element 3 through the sector gear 4 and a transmission mechanism 8 comprising a gear train which is carried by the body 1 and which includes a pinion 9 meshing with the sector gear 4, a crown gear 10 integrally fixed on the pinion 9, and a pinion 11 integrally formed on the pointer shaft 6a and meshing with the crown gear 10.

Fixed in the body 1 are a pair of spring-retaining pins 1 2 and 13 which are spaced apart from each other and are disposed on opposite sides of an intermediate arm portion 4a of the sector gear 4, and the intermediate arm portion 4a has a springengaging device 1 4 fixed to it near the end which is mounted on the shaft 2. As shown in Figures 2 and 3, this spring-engaging device 14 has a projection 1 4a which is provided on opposite sides thereof with substantially V-shaped spring receiving grooves 1 4b and 1 4b'. Also fixed in the body 1 are projections 1 5 and 1 6 located at positions adjacent and below the pins 1 2 and 1 3.

The respective ends of a pair of leaf springs 1 7 and 1 8 are wound around and supported by the pins 12 and 13, respectively, the springs 1 7 and 1 8 extending from the pins over the projections 1 5 and 1 6 and into engagement at their free ends with the grooves 1 4b and 1 4b' of the springengaging device 14. These leaf springs 1 7 and 18 act on the spring-engaging device 14 in opposition to each other, each tending to urge the sector gear 4 to rotate about the shaft 2 in a direction opposite to the other.

One of the springs 1 7 is slightly stronger than the other spring 18, and its extreme free end portion projects beyond the spring-engaging device 14 towards the shaft 2. As shown in Figure 1 , the forward end of a screw 1 9, constituting a fine adjusting means, abuts against the extreme free end portion of the spring 1 7 from the side towards which the spring 17 is moved by its own resilience. The screw 1 9 is screwed in a tapped bore in the body 1 so as to be linearly adjustable in the direction of resistance to the resilient force of the leaf spring 17.As a result, the position of the screw 1 9 determines the rest or neutral position of the sector gear 4, and hence of the measuring element 3 aiso, and by linear adjustment of the screw 19 this neutral position can be finely adjusted until the pointer 6 indicates the zero position of the dial 7. In other words, the zero position of the instrument is readily adjustable so that accurate measurements are always ensured.

In this example the spring-engaging device 14 provided with the V-shaped grooves 1 4b and 1 4b' is formed separately from and fixed to the sector gear 4. In an alternative construction, however, the spring-engaging device may comprise a projection integrally formed with the sector gear 4, or a simple pin. In short, any construction suitably engageable with the springs 1 7 and 1 8 may be sufficient. However, the provision of the spring receiving grooves 1 4b and 1 4b' is advantageous in that the springs 1 7 and 1 8 are positively engaged therewith and are not easily displaced. Additionally, the springs 1 7 and 1 8 need not necessarily be leaf springs, but may be wire springs. However, the use of leaf springs is advantageous in that the extreme free end portion of the spring 17 can be positively abutted against the forward end of the screw 1 9.

Further, in present example the fine adjusting means for the zero position is the screw 1 9. If desired, however, this screw may be replaced by an eccentric cam or the like. In the case of an eccentric cam, the eccentric outer periphery thereof is arranged to abut against the free end portion of the spring 1 7, and rotation of the cam provides the adjustment. Furthermore, the portion of the fine adjusting means which abuts against the spring need not necessarily be the forward end portion, but may be an intermediate portion.

However, the use of the forward end portion is advantageous in that the installation of the adjusting means is easier.

Claims (5)

1. An instrument in which a rotational displacement of a measuring element is amplified and transmitted to a measurement indicator by a mechanism which includes a sector gear mounted on the body of the instrument so that it moves in response to rotational displacement of the measuring element, characterised in that the measuring element is biassed to a substantially central zero position by means of a pair of springs which act on the sector gear in opposition to each other, and in that the two springs are of different strengths and the body carries a finely adjustable stop for engaging the stronger of the two springs to limit the extent to which the sector gear can be moved by the stronger spring against the action of the weaker spring.

2. An instrument according to claim 1, in which the springs are leaf springs.

3. An instrument according to claim 1 or claim 2, in which the two springs act on the sector gear through a spring-engaging portion on the sector gear and having a pair of V-shaped grooves in which the springs seat.

4. An instrument according to any one of the preceding claims, in which the finely adjustable stop comprises an adjusting screw.

5. An instrument according to claim 1, substantially as described with reference to the accompanying drawings.