
MarMotion high-precision rotary stroke bearings consist of the cylindrical shaft and bush, which act as guiding elements, and the steel balls, which act as the rolling bearing elements. The balls are held in a brass or plastic smooth-moving tube where they roll easily. | |
The guide shaft, balls and guide bush are specially hardened and micro-finished. The structure is stabilized through careful application of heat treatment. The balls roll non-positively between the bush and shaft under a preloading tension. Linear, rotary and combined movements are possible. The frictional connection of the balls means the cage moves in accordance with the laws of kinematics. |

Preloading must be set with great precision to ensure problem-free operation. This is achieved and defined using the manufacturing tolerances. Preloading value v is the difference between the size of the inside diameter of the guide bush d1 and the distance between two balls lying opposite each other and touching the shaft.
Recommended preloading Preloading values:
These preloading values ensure that the rotary stroke bearing has both excellent rigidity and smooth running. In the case of orders for complete rotary stroke bearings consisting of a guide shaft, ball cage and guide bush, the components are paired in the factory. This ensures optimum and uniform preloading. Due to the range of scatter of the manufacturing tolerances (IT 3), indiscriminate pairing of shafts, ball cages and bushes can result in unfavorable preloading values which deviate from those listed in the table. The rotary stroke bearings of the Mini Range should always be ordered in pairs. The significance of the preloading A very low preloading value means that the rotary stroke bearing runs smoothly but rigidity is limited. A higher preloading value increases loading capacity and rigidity. A very high preloading value leads to the rotary stroke bearing running less smoothly. If the preloading value is too high, the run will be rough and stiff. The rotary stroke bearing can also be overloaded by excessive surface pressure. This is prevented by the narrow manufacturing tolerances of the rotary stroke bearing. It must therefore be ensured that the guide bush is not deformed when being installed. The guide bush should thus not be pressed into the location bore or secured with pressure screws.
Optimum preloading
The diagram shows the radial offset of a rotary stroke bearing as a function of the preloading value and the radial load. For a given radial force, the offset with a small preloading value is relatively large (the guide is soft). With high preloading values, however, the offset is significantly smaller for the same radial force (the guide is rigid). Taking into account the Hertzian stress, manufacturing tolerances and deformation of components during installation and operation of the rotary stroke bearing, and also taking into account the most favorable resilience conditions for the rotary stroke bearing, a value of δR= 0.5 v has been taken as the basis for the computations. This satisfies the call for “high-precision�. This condition is met for the indicated “specific rated loads� C10. Matching of the shaft and ball diameters On the other hand, a large number of smaller balls results in better vibration dampening than a small number of large balls. For this reason, and in order to save space, the smaller sized ball is often preferred. Moreover, with a low degree of roughness and greater geometrical accuracy of form for the rolling element, the ball diameter becomes relatively unimportant for the running characteristics. The shaft and ball diameters of the MarMotion high-precision rotary stroke bearings have been matched to optimum effect and the optimum number of balls defined on the basis of thorough testing. |

The coefficients of friction µ apply for the start-up run and movement alike. Influencing variables:
The MarMotion high-precision rotary stroke bearings run free of stick-slip. The following coefficients of friction apply to radial load:
The rolling resistance of a rotary stroke bearing derives from the inner load caused by preloading and from the influence of outside radial forces. With a low radial load, the preloading and cage friction components predominate. The coefficient of friction µ thus increases as the radial load decreases. Consequently, when there is a small radial load and extremely smooth motion is required, a low preloading value must be used. |