GB2312732A - Mountings for rotating shafts in a position indicator - Google Patents

Abstract

A position indicator comprises a trackerball 11 which engages two encoder shafts 1, 13 disposed substantially at right angles to each other. The shafts are rotatably mounted in bearings 3 with a resilient sleeve or 0-ring (5, figure 1) between each shaft and its bearing(s). The sleeve or 0-ring reduces noise and vibration.

Description

IMPROVEMENTS IN AND RELATING TO MOUNTINGS FOR ROTATING SHAFTS Field of the Invention This invention concems improvements in and relating to mountings for rotating shafts, particularly but not exclusively of the type used to mount trackerballs or other position indicator devices.

Background to the Invention Tracker balls form an alternative to a mouse for moving a cursor on a screen as a result of manual movement of the trackerball. Thus the ball's rotation, left, right, up and down is reflected in the movement of the cursor. Such balls find applications where a mouse is not practical, such as lap top computers.

A typical trackerball comprises a ball held in contact with and a pair of encoder shafts mounted substantially at right angles to each other and in the same plane. The periphery of the ball is in contact with each of the shafts. Thus the rotation of the ball is transferred into a rotation of the shafts. Movement of the ball perpendicular to one shaft causes rotation of that shaft only and similarly movement perpendicular to the other shaft rotates that shaft only. Intermediate directions of movement produce a degree of rotation in the two shafts depending upon the angle of movement of the trackerball. The shafts are connected to transducers or transponders which provide electrical signals which are converted into a corresponding position of the cursor on the screen.

In prior art systems the shafts are mounted in ball bearings. Unfortunately noise and vibration originating in the rolling surface between the ball and the encoder shafts is transmitted to other parts of the assembly and housing through the bearings in such systems. This is undesirable as it does not give the user the impression of a precision engineered product.

This so-called "feel" is most important to the user. Typically, the ball is made from a very hard-wearing synthetic material, such as a phenolic resin and the encoder shafts are made of metal such as stainless steel. In the absence of any vibration damping means, this results in audible "noise" when the ball is rotated as well as tactile vibrations, or tactile noise, felt by the operator through the ball. Any slight irregularities in the ball, shafts or bearings are easily perceived, in use, by the operator.

It is one object of the present invention to reduce one or both of these sources of "noise".

Prior art attempts to overcome this problem have included providing a rubber sleeve at the interface between the encoder shaft and the ball. Unfortunately such sleeves tend to wear and come loose on the shaft. Furthermore, when the ball is left in one position the sleeve tends to develop a semi-permanent flat surface which makes subsequent rolling motion unacceptably bumpy. Given the very sensitive nature of the feel required in this type of application, feedback through the trackerball to the user of this detrimental effect is not tolerable. In addition, the high co-efficient of friction between the ball and rubber tyre means that the ball does not roll easily and the positive "feel" of the device is lost.

Alternative attempts to solve the problem have included the use of plastic bearings and/or providing the mounting of the entire trackerball frame in noise absorbent mounts. Drawbacks in other areas exist with such approaches, such as increased manufacturing costs, and/or reduced or unknown life expectancy.

Finally, some companies have dispensed with ball bearings altogether and incorporated mby or PTFE fixed bearings. However, this type of sliding engagement remains unproven in terms of bearing life, long term reliability and performance in hostile environments.

Amongst the aims of the present invention is to provide a low noise and vibration mounting system for encoder shafts which is both reliable and cost effective.

Summary of the Invention According to the present invention in its broadest sense, there is provided a position indicator device comprising a ball which, in use, is held in contact with two encoder shafts disposed substantially at right angles to each other, said shafts being rotatably mounted in bearings, characterised in that the bearings incorporate a resilient sleeve between the shaft and the bearing.

This arrangement retains all the rigidity of prior art systems whilst providing the required noise and vibration damping. In this way, noise and vibration introduced to the shaft at the contact between the trackerball and shaft is not transmitted any further than the shaft itself. Noise and vibration elsewhere in the assembly are thus vastly reduced. Moreover, this type of system has the required feet and smooth running whilst being virtually inaudible under normal conditions of use.

Preferably the sleeve takes the form of an O-ring. O-rings are inexpensive and readily available in a wide variety of materials.

In a particularly preferred embodiment, the resilient sleeve is mounted in an annular groove or channel located in the outer radial circumference of the shaft, such that the sleeve is prevented from moving along the shaft and is effectively captive within the groove.

Thus there is no direct contact between the shaft and the mounting. By locating the sleeve in a groove, it is effectively anchored beneath the bearing.

Preferably the sleeve is made from nitrile rubber.

Brief Description of the Drawings The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a detail of a mounting according to a first embodiment of the invention; Figure 2 shows the combined arrangement of a pair of encoder shafts and trackerball according to the invention, one of the bearings on shaft 13 is not shown for reasons of clarity; Figure 3 shows the spatial relationship between the ball and one of the encoder shafts.

Description of the Preferred Embodiments The present embodiments represent currently the best ways known to the Applicant of putting the invention into practice, but they are not the only ways in which this could be achieved. They are illustrated, and they will now be described, by way of example only.

As illustrated in Figure 1 of the encoder shaft (1) is mounted in a ball bearing race (3). However, the interface between the shaft and ball bearing race is provided by an O-ring (5) of suitable shock absorbing ability. To aid in the retention of the 0ring in the desired location it is preferably accommodated within an annular groove (7). A clear space (9) between the shaft (1) and ball bearing race (3) is maintained at all times. In this way vibration and noise in the shaft is substantially absorbed by the O-ring (5) and not communicated to the ball bearing race and hence on to other parts of the assembly and vice versa.

Referring to Figure 2 the movements of the trackerball (11) are communicated to the shafts (1) and (13) at interfaces (15) and (17) respectively.

Similar mountings involving O-rings are provided at locations (19), (21) and (23).

A wide variety of materials can be used for the resilient sleeve or O-ring.

These include natural and synthetic rubbers such as nitrile rubber, chloroprene, or neoprene. A typical hardness value would be 70 IRHD and a typical material would be KYNAR (Registered Trade Mark). This material has specific properties which make it particularly suitable for this application In a preferred embodiment, the vibration absorbing component takes the form of a sleeve around the shaft, rather than an O-ring. This is particularly applicable in higher load applications, where the forces are spread across the entire width of the sleeve (not illustrated).

In this type of application, the object is to reduce audible noise from the trackerball and also tactile vibrations felt by the operator through the ball. The reduction gained in both types of noise with this invention is a significant improvement over the prior art.

Previous designs have had clearly audible levels of noise in a normal working environment, whilst the present invention renders this noise virtually inaudible against normal background noise. Because the sound pressure levels concerned are close to background noise levels, it is not possible to measure them without use of an anechoic chamber. In previous designs, the tactile noise felt by the operator has been easily noticeable while the invention is capable of reducing this noise to a very low level where it is almost unnoticeable.

The particular applications for which this noise and vibration reducing method are aimed are specialised. In particular, the loading on the shafts is light and primarily in the radial direction, also the centreline of the rotating shaft is allowed a small but limited displacement from the centreline of the bearing (in the case of a trackerball a typical case the maximum allowable displacement is 0.1 mum). Also, the required isolation interface is between the rotating shaft and the supporting structure rather than between one end of the shaft and the other or between the shaft and another rotating element. Also the freedom of rotation of the shafts must not be constrained by the noise reduction method.

The particular constraints of this type of system are adequately satisfied by the mounting system now proposed.

It will be appreciated that this type of vibration damping system will have other uses in related and unrelated fields.

In the case of related fields, it could find application in a computer mouse or in a joystick. This could extend to the type of toggle switch used to control CCTV cameras.

Because the shaft, vibration damping means and inner port of the bearing rotate as one, this means that this type of assembly will have a variety of applications outside the computer field. For the avoidance of doubt, this invention extends to the general principles described herein and should not be considered application dependent.

Claims (5)

Claims

1. A position indicator device comprising a ball which, in use, is held in contact with two encoder shafts disposed substantially at right angles to each other, said shafts being rotatably mounted in bearings, charactensed In that the bearing incorporates a resilient sleeve between the shaft and the bearing.

2. A position indicator device as claimed in Claim 1 wherein the sleeve takes the form of an O-ring.

3. A position indicator device as claimed in Claim 1 or Claim 2 wherein the resilient sleeve is mounted in an annular grooved channel located in the outer radial circumference of the shaft such that the sleeve is prevented from moving along the shaft and is effectively captive within the groove.

4. A position indicator device as claimed in any preceding Claim wherein the sleeve is made from nitrile rubber.

5. A position indicator device substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.