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-        42 M a r M o t i o n . H i g h - p re c i s i o n ro t a r y s t ro k e b e a r i n g s

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5. Computation of the Rotary Stroke Bearing

Fig. 19                         The computation formulae are intended to assist the designer in determining the rotary
                                stroke bearing which is suited for a given design task.

                                Computation of the rotary stroke bearings is based in principle on the laws governing
                                ball bearings. However, they differ essentially from the latter in that they allow
                                movement in two degrees of freedom, so that length dimensions and acceleration
                                values must be taken into account. Furthermore, the internal osculating conditions
                                differ substantially from those of most ball bearings.

                                The following features and characteristics are of primary importance in determining a
                                suitable rotary stroke bearing:
                                • Freedom from backlash
                                • Guiding accuracy
                                • Smooth running
                                • Loading capacity
                                • Stroke and rotary frequency
                                • Service life
                                • Dimensions

                                The following variables are to be determined:
                                • Shaft diameter dw
                                • Bush length l1, l3
                                • Cage length l2

                                Nominal diameter dw and the cage type determine specific loading capacity C10.
                                The lengths of the guide bush and ball cage determine the contact length of the rotary
                                stroke bearing. These values are used to calculate the operational loading capacity of the
                                rotary stroke bearing.

Fig. 20                         5.1 Stroke path and contact lengths

     dw   N 500  E [mm]  N 502  The ball contact lengths are determined by the mutual positions of guide bush, ball cage
   [mm]                         and guide shaft at the end of the stroke. The dimensions l1 and l2 lead to a distinction
             -   N 501      5   between different operating modes.
   3         -              8
   4- 5      -      -     10    Open rotary stroke bearing (Fig. 19)
   6- 8     12     10     12    Cage length l2 equal to or greater than bush length I1.
 10 - 12     -     12     15    Stroke: H = 2 (I2 – l1)
 14 - 16    12     15     15
 18 - 20    12     18       -   Closed rotary stroke bearing (Fig. 20)
 25         15     18       -   Bush length l3 greater than cage length l2, ball cage remains within the bush.
 32 - 40     -     20       -   Stroke: H = 2 (I3 - l2)
 50 - 63     -     22       -
 80 -100           30           A contact length E which remains constant across the entire stroke (in every stroke
                   45           position) is desirable for both operating modes. This is always fulfilled for closed rotary
Fig. 21                         stroke bearings. In the case of open rotary stroke bearings, the bush should be flush
                                with the cage in the end stroke positions. If the bush extends beyond the end of the
                                cage, this shortens the contact length and thus reduces the loading capacity of the
                                rotary stroke bearing in this stroke position.

                                The minimum permissible contact length E must be determined by calculating the
                                loading capacity. The loading capacities of the ball cages can be taken as guideline values.

                                Reference contact length e
                                In order to compute a rotary stroke bearing, it is necessary to determine the reference
                                contact lengths e = contact length with unfavorable load distribution.
                                (see Fig. 23 and Fig. 24)

                                With a small load
                                The table (Fig. 21) shows the recommended minimum contact length E depending
                                on dw for accurate guiding using the rotary stroke bearing without significant load.

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