Principle Designs

5.5. Principle Designs

After defining the primary goals for testing bearings with high dynamic loads, steps 2 and 3 from fig. 5.2 have to be tackled. As already mentioned, these two steps cannot always be strictly separated from each other.

The first and foremost component of interest is the specimen itself. To achieve syn-chronisation of force and IR angle two functional options exist:

1. Regulation of the force and the timing, fig. 5.5 (a) 2. Controlled application of force, fig. 5.5 (b)

The design proposal 1 is simple and straight forward. It can be effortlessly established and solely the rotation speed of the specimen shaft is needed. This implies that the position of the shaft is known, however, only the given rotation speed of the drive can be known. The effects of wear, elasticity and most importantly friction upon the shaft’s rotational movement are somewhat uncertain. The aim of synchronisation of impact time and shaft position would be jeopardised by such a design. Proposal 2 is capable of avoiding such uncertainties, provided the sensitivity of the employed means of measuring is high enough. The drawback of this system are higher costs or / and effort.

During the first iteration of this approach a maximum force of +60 kN per bearing was projected and the expected power loss and torque due friction were estimated as high as 370 W and 780 Nmm respectively per bearing at the time of maximum force. For these estimations catalogue bearings with static load were assumed. Since additional friction in the instant of impact cannot be dismissed even higher torque must be assumed. As such heavy torque will involuntarily lead to distortions of the shaft’s rotation, no choice remains other than to work with a controlled application of force as shown in fig. 5.5 (b).

2πf = ω

fig. 5.5.: The basic step 2 design proposals available

CHAPTER 5. DESIGN AND DEVELOPMENT OF A TEST RIG FOR DYNAMIC TESTING UNDER HIGHLY TIME-VARIANT LOAD CHANGES

In order to control the timing of force application several principle designs are avail-able. The order of presentation in fig.5.6 does not reflect the chronological order of evaluation. In fact proposals 3.2 and 3.3 are well established designs used in different applications and are industrial standard at present.

As chapter 5.6.1 will show, alternative designs had to be considered as well. Fig.5.6 (a) exemplary shows pure mechanical designs. The advantage of purely mechanical designs are their high speeds. The most crude design of such a mechanic would be the depicted one, consisting of gears rotating a cam which exerts a force upon the specimen.

It goes without saying that a practical design would probably not consist of cranks or timing belts but rather inelastic elements like timing chains. A variation would be the use of shifter shafts as known from high speed machine tools. Thereby a shaft is turned synchronously with the specimen shaft. The shaft has one or several grooves which a pin-lever is guided. Depending on the grooves position the tool (in this case the force applying system) is displaced. In the present case this would be a simple up and down movement. In order to cope with different speeds different diameters can be used. Two considerable problems, however, arise in all designs: Wear and capable loads. Calculations for the cam showed that the contact stresses on the tip of a cam would be in the magnitude of several gigapascal, no matter how wide the cam would be.

The shape of the cam tip is restricted by the necessary time demanded. Moreover wear cannot be easily fought off. As modern combustion engines have considerable service lives, a testing time in the order of more than 10⁸ revolutions may become necessary.

One possible answer would be a casing around the position of cam / bearing contact.

This would have to be experimentally evaluated and is beyond the scope of this work.

Another, totally different set of mechanical designs is the use of event triggered levers (5.6 (b)). In these cases the specimen shaft levitates a trigger, which sets mechanical parts into motion resulting in a weight being levered onto the bearing. Since the pos-sibilities of the shaft to lever large masses is restricted, only external weights or spring systems are capable to exert enough force. Since the necessary masses for the load cannot be instantly retracted by any means, these methods are simply not feasible.

The third option is the use of electromechanical or electrical activated actuators (c).

This option is widely used in industries and thus knowledge and availability is wide spread. Common feature of all these designs is that electronic controlled actuators exert given forces upon the bearings. The use of controllers also give enhanced flexibility by easily enabling additional operations, such as intentional delays, programmable force-time curves, etc. The most important advantage is the unification of all controlled or electronic dependent parts of the test-rig in digital control units, provided enough com-putational power is available.

Another crucial point regarding the shaft is it’s arrangement of bearings. The number and arrangement of the bearings define the size, weight and additional stresses seen by the shaft and thus at the raceways of the shaft. As the number of the active bearings increase, i.e. the number of the bearings connected to the actuator, the required actuator force increases. Also the size and inertia of the shaft increases. While a high inertia itself would reduce deceleration due to friction, it also increases necessary driving power at

5.5. PRINCIPLE DESIGNS

(a) Design 1

F

(b) Design 2

Controller F

M D Actuator

(c) Designs 3.1, 3.2 and 3.3

fig. 5.6.: The step 3 solutions found

CHAPTER 5. DESIGN AND DEVELOPMENT OF A TEST RIG FOR DYNAMIC TESTING UNDER HIGHLY TIME-VARIANT LOAD CHANGES

reduced handling capability due to size. Small shafts with very few test bearings will extend testing time as more testing has to be conducted for statistics. The bending of the shaft and the stiffness of the bearings have to be taken into account as well.

Fig. 5.7 (a) to (d) show four basic types of arrangements. The crossed orange squares represent potential locations of test bearings while green squares represent support bear-ings. The red line is the anticipated deflection curve of the shaft at static load with the dotted centre line as zero deflection.

F

fig. 5.7.: The step 2 & 3 arrangements for bearing arrangement

Arrangement (a) is a simple and widespread arrangement which is realised in the BTR (see 5.2.6) so that an appropriate choice of shaft diameter for the rig would allow for later friction measuring on the BTR. The obvious advantage is the simplicity of this arrangement. Also the necessary actuator force is little, so are the deflections of bearings and shaft, especially when large support bearings with high stiffness are used.

On the other hand the number of tests to be conducted is high, which means that