MECHANICAL MEASUREMENTS

M E A S U R E M E N T OF SMALL S T A R T I N G M O M E N T S

Yu. M. K h a n d e l ' s m a n , V. V. D o k u c h a l o v a , and A.

Translated from Izmeri tel 'naya Tekhnika, No. 2, pp. 17-19, February, 1982

S. M i k h a i l y u k

When mechanisms with trigger regulators are made it is important to know not only the tdn~tic friction mo- ment of the bearings but also their starting moment. This problem is becoming increasingly important in connection with the developing tendency to reduce the size of modern instruments. Friction in the bearings of measuring instru- ments with rotating parts has a direct effect on their accuracy.

d

Fig. 1

r 6 5

pt

Fig. 2

The pendulum and the calibrated spring methods have be- come the most widely used of existing mechanical and electri- cal methods for measuring the starting moment.

The schematic of an equipment with a pendulum drive, used by the NIIChasprom (Scientific Research Institute of the Watchmaking Industry) for checking starting moments in small diameter jewel bearings is shown in Fig. 1. This equipment is sufficiently universal and may find wide application. The start- ing moment is determined in a set consisting of two bearings 1 and two pivots 2. The pivots are fixed to spindle 3 which car- ties unbalanced pendulum 4, and the bearings are mounted in capsule 5. (A reversed arrangement is possible with the pivots in a capsule and the bearings on the spindle.) The value of the starting moment depends on the geometrical position of the driven parts of the system.

The capsule with the bearings is made to rotate at a con- stant speed. The driven parts of the system consisting of the spin- dle with the pivots and the pendulum are displaced by the rota- ting bearings owing to the friction force. The spindle is rotated until the moment provided by the pendulum equals the friction moment in the bearings. At the instant the spindle stops rotat- ing together with the bearings, scale 6 is used for reading the "starting" angle a of the spindle, i.e. the angle of the common displacement of the system consisting of the capsule and the spindle. The value of the starting moment of the set of bearings is determined by the rotation angle of an artificially produced pendulum (it is assumed that the center of gravity of the spindle with the pivots but without the pendulum is located on the axis of rotation) and is found from the expression

Mst = P I sin a, (1)

where Mst is the starting moment, g. cm; P is the weight of the pendulum in the tested medium, g; l is the distance from the axis of rotation to the center of gravity of the pendu- lum, cm; a is the "starting" angle of the spindle, deg.

In principle this method makes it possible to determine an arbitrarily small starting moment of the bearings. However, the measurement of starting moments smaller than 10"4 g. cm for technical reasons encounters difficulties, the most important of which consists of the difficulty in obtaining an accurate balancing of the spindle.

The set of tested bearings is fixed in a mounting chamber (Fig. 2) , which consists of base 1 with a flange, glass cylinder 2, top cover 3 used for mounting the vibrator, capsule 4, the lower cover 5 which carries the capsule, and

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spindle 6. As we have already stated, the driving e lement can consist of the spindle fitted ei ther with pivots or bea r -

ings. In the first instance rollers 1 (Fig. 3) are used with co l le t chucks for fixing pivots 2, and bearings 3 are pressed

into exchangeable inserts 4. I f the tested bearing is provided with a thrus t -p iece 5, i t is also p laced in the insert. Be-

fore fixing the set of bearings in the mounting chamber and screwing pendulum 6 into the spindle, the moving parts

f 2 ~ r 5

Fig, 3

~ ,r 5 4

Fig. 4

are careful ly balanced s ta t ica l ly by means of adjusting screws 7 p laced along the c i rcumference of the spindle, since error in determining small starting moments depends to a great ex - tent on the value of the res idua l imba lauce of the system's

moving parts.

The balancing of the spindle with the pivots is done

on a balancing device with ruby blades which is used in watch factories for balancing purposes. A spindle with bearings is balanced on pivots. The balancing error in this case is con-

sider ably higher.

In balancing reversed couples, bearings 1 (Fig. 4) with their

thrust-pieces 2 are mounted on spindle 3, and the inserts 4 of the capsule are fi t ted with col le t chucks to take pivots 5. Such a con- struction of the mounting chamber makes i t possible to test bearings of different types by changing only inserts and the spindle.

The mounting chamber is sealed he rmet ica l ly , which makes

i t possible to de te rmine the starting moment not only in air but a l - so in other media , including vacuum.

For testing at various tempera ture conditions provisions are made for heat ing the chamber and controll ing the tempera ture by means of a thermistor transducer mounted at the base of the chamber. Having assembled the set of bearings, the mounting chamber is p laced on a rotat ing device fixed to a facep la te with a drive through a reduction gear. The rotat ing device makes i t possible speedi ly to displace the mounting chamber with respect to the faceplate . The beginning of the r e l a t ive d isp lacement (starting) is set visual ly by means of marks on the spindle and a micro- scope motmted on the rotat ing device. The starting angle is read off a dial with a Vernier scale which is fixed to the facepla te and has an error not exceeding 0.1". The unbalanced spindle is set to the zero position by means of a v ibra- tor which is fixed to the mounting chamber. When the vibrator is connected the friction in the bearings is reduced and the pendulum assumes its ex t reme lower position. In addition to using the vibrator, all possible measures are t a - ken to avoid any extraneous vibrations, since even the most insignif icant of these may distort considerably the meas - urement results. The facepla te and cant i lever bracket are made of massive castings, and an external drive is used. The equipment is mounted on a concrete bed with an insulated foundation. The facepla te is made to drive the spin-

die at a low rotat ion speed of the order of 1-45 ~ per minute, with the same object in view and in order to e l imina te any hydro- and aerodynamic effects.

It should be remembered in using the equipment that when the pivot is mounted on the spindle the evaluat ion

of Mst is made at random points of contact of the pivot with the bearings. When the mounting chamber is rotated the points of contact in the bearings change, but those in the pivots remain the same. When the bearings are mounted on the spindle reversed conditions will obtain. The contact surfaces of the bearings are normal ly studied, since a vis- ual examinat ion of the contact surfaces of smal l journal bearings is v i r tual ly impossible without damaging them, whereas any defects in the pivots are easy to observe.

It has been established expe t imen ta l ly that i t is sufficient to measure jewel bearings at 20 points in order to e -

va lua te their max imum Mst.

It is advisable to measure Mst at fixed points when its re lat ion to different lubricants, med ia and tempera ture is evaluated by means of the equipment. The mounting chamber under the above conditions is returned after each measurement to its in i t i a l position by means of the rotat ing device. The measurement error does not then exceed 10%.

The effect of various overloads on the starting m o m e n t of bearings should also be measured at fixed points. The mounting chamber is constructed in such a manner that i t can easi ly be fixed to vibrat ion racks, centrifuges and other test machines. Moreover, the chamber provides any required test conditions ( tempera ture , medium, etc.) .

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