OVERLOAD PROTECTION DEVICE FOR TORQUE MEASURING TRANSDUCERS

Abstract: Overload protection device for torque measuring transducers comprising a measuring axle (1) intended for a certain maxi­ mum measurement range. According to the invention the device is surrounded by a housing (3) which is preferably jour­ nalled riding on the measuring axle, but being prevented from rotating by means of a link to a fixed point. A certain length of the measuring axle in the housing is used as mea­ suring distance and at either end thereof a larger gear wheel (4,5) is attached free of backlash. These gear wheels are in engagement, with a well-defined backlash, with re­ spective smaller gear wheels (6,7) which are each fixed to one end of a second axle (2) journalled in the housing. When applying a torque on the measuring axle which with a certain margin exceeds said measurement range, the twist of the mea­ suring axle between the larger gear wheels will arrive at such a value that the backlash between the teeth is elimi­ nated, thus causing the takeover of overload by the second axle 2. The input torque on the measuring axle is geared down to the second axle, which is therefore insignificantly twisted upon additional overload, thus effectively protect­ ing the measuring axle against excess overload.

Description: The invention relates to an overload protection device for torque measuring transducers according to the precharacterising part of Claim 1. The device is preferably meant for torque measuring in rotating axles. Transducer for measuring the torque in rotating axles are designed according to different principles. In some transducers the twist of the axle is measured over a certain axial length by the phase difference between the signals from two gear rings. In other transducers, the shearing stress in the axle is measured by different methods, for example by means of strain gauges, whereby the strain gauge and the signal processing circuit, mounted on the axle, are supplied via a rotating transformer, the measuring signal being transmitted in a wireless manner. In still other transducers, the shearing stress is measured by magneto-elastic methods, such as, for example, in SE-B-167387, in which the magnetic anisotrophy in the axle, caused by the shearing stress, is measured with the aid of a system of magnetizing and sensing magnetic poles arranged around the axle. In a magneto-elastic transducer according to US-A-4,506,554, a measuring axle is provided with a measuring sleeve which, in two separate zones, has been formed with slots which in one zone has a +45° deviation from the axial direction and in the other zone a -45° deviation from the axial direction. The difference in reluctance between the two zones, arising when applying a torque, is sensed by a surrounding coil system. In all of the afore-mentioned transducers, the demands concerning the choice of the stress level in the measuring axle at the upper limit of the measurement range are contradictory: To obtain a good signal/noise ratio and a small temperature dependence, a high maximum measuring stress should be chosen, but to obtain a good overload capacity without the zero point of the signal being changed, a low maximum measuring stress must be chosen, resulting in the signal/noise ratio and the temperature dependence being necessarily adversely affected. Thus, in prior art transducers there is a conflict between the demand for a high measuring stress in the axle to obtain a good signal/noise ratio as well as a low temperature dependence and the demand for a low measuring stress in the axle to obtain a good overload capacity. The invention of claim 1 aims at designing an overload protection device for torque measuring transducers of the above-mentioned kind which enables the combination of high stress in the measuring axle within the measuring range and a high overload protection against a torque exceeding the measuring range. To achieve this aim the invention of claim 1 suggests an overload protection device for torque measuring transducers according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. The invention of claim 1 eliminates the above described conflict with prior art transducers by constructing the measuring axle with such a slender dimension that the maximum measuring stress brings about a satisfactory twist of the measuring axle while at the same time achieving a high overload capacity by allowing ― at a certain twist of the measuring axle ― a stronger structural member to assume the greater part of the torque. The device will now be described in greater detail with reference to the accompanying drawings showing ― by way of example ― in Figure 1 an embodiment of a device according to the invention which, when the measuring axle is loaded above a certain limit, takes over the greater part of the excess load, and Figure 2 an alternative embodiment of the engagement of teeth between two gear wheels in a device according to Figure 1 to obtain an accurately-defined backlash. In the preferred embodiment of the invention shown in Figure 1, the torque-loaded measuring axle 1 and the heavier structural member which is to take up the main part of the overload, in this case in the form of a second axle 2, are illustrated. The second axle 2 is mounted in a housing 3, which is suitably journalled riding on the measuring axle 1. The housing is prevented from rotating by means of a link to a fixed point (not shown). On the measuring axle 1 two larger gear wheels 4 and 5 are attached, without backlash, at such a distance from one another that a reasonable twist between the gear wheels is obtained when the maximum measuring load has been reached thus also permitting satisfactory space between the gear wheels to accommodate a torque transducer, for example according to the above-mentioned US-A-4,506,554. On the second axle 2, the overload axle, two smaller gear wheels 6, 7 are arranged without backlash, preferably milled out directly in the axle 2. These gear wheels 6, 7 are in engagement with the respective gear wheels 4, 5 with a well-defined total backlash. This backlash is dimensioned so as to be eliminated by the twist of the measuring axle 1 between the gear wheels 4, 5 upon application of a torque which, with a suitable margin, exceeds the maximum measuring torque. If Figure 1 were a reproduction on a 1: 1 scale, the total backlash would be about 1 mm, half of it in either torque direction. A practicable backlash may be as small as ± 0.1 mm. The backlash may be distributed equally between the two engagements of teeth. To facilitate adjusting the correct neutral position, so as to obtain the same backlash in both torque directions, either of the engagements 4, 6 or 5, 7 can be made as free from backlash as possible while the other engagement of teeth can be made, for example, as shown in Figure 2 with thinner teeth 8 in the gear wheels 6, 7 of the overload axle than the corresponding teeth 9 in the gear wheels 4, 5 of the measuring axle 1. The second axle 2, the overload axle, is described above as the stronger structural member which is to take over the major part of the overload. However, in Figure 1 this axle is shown with only insignificantly larger diameter than that of the measuring axle 1. This dimensioning is sufficient due to the fact that, when the backlash has been eliminated upon overload, the torque in the second axle 2 is geared down. With a relationship between the gear wheels as shown in Figure 1, the torque is geared down to about one-fourth, resulting in insignificant twisting of the second axle 2. The additional load on the measuring axle 1, caused by the overload, is therefore very small.

Claims: 1. Overload protection device for torque measuring transducers comprising a measuring axle (1) intended for a certain maximum measurement range, characterized in that the measuring axle (1) is rotatably journalled in a housing (3), in which housing part of the measuring axle is arranged to define a measuring distance, the measuring axle at each end of the measuring distance is provided with a larger gear wheel (4, 5) attached to the axle without backlash, said larger gear wheels are in engagement, with a certain well-defined total backlash, with respective smaller gear wheels (6, 7) which are arranged without backlash on a second axle (2) being also rotatably journalled in the housing (3), and when applying a load on the measuring axle which, with a certain margin, exceeds the measurement range, the twist in the measuring axle obtained over the measuring distance between the two larger gear wheels (4, 5) is of such magnitude that the well-defined total backlash between the teeth has been eliminated, the measuring axle thus being relieved by said second axle. 2. Overload protection device according to Claim 1, characterized in that the total backlash is provided in one engagement of gears (4, 6; 5, 7) only while the other engagement of gears is designed with as small a backlash as possible. 3. Overload protection device according to any of Claims 1 or 2, characterized in that the smaller gear wheels (6, 7) on the second axle (2) are manufactured by milling them out in said second axle.

Inventors: AMINDER,CHRISTER, NORDVALL,JAN O.
Applicants: ASEA BROWN BOVERI AB
Application year: 1988
Main classification: G01L3
IPC codes: G01L3