Rotary Stroke Bearings

Rotary Stroke Bearings
Design and functions
EN
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.

Rotary Stroke Bearings
Preloading
EN

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.


v = dw + 2k – d1 [mm]
d1 = dw + 2k – v [mm]

Recommended preloading
The preloading values given in the following table are recommended for most applications. These values are based on theoretical knowledge and practical experience.

Preloading values:

dw [mm] v [µm]
2,5/3 0.5–2
4/5 1–3
6/8 2–4
10/12 3–5
14/25 4–7
30/42 4–8
50/63 6–10
80/100 8–12

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
Preloading guarantees that the MarMotion high-precision rotary stroke bearing has absolutely no backlash. In the case of applications where special conditions have to be satisfied, the required preloading value can be specified at the time of ordering.

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.

Stationary guide bush

PR = radial force, acting on the shaft
P0 = force acting on the ball at the load apex
Pi = force on a ball outside the load apex
δR = radial offset of the bush and shaft axes
v = preloading

Optimum preloading
If the rotary stroke bearing is loaded radially with a force PR, the guide bush axis and shaft axis are displaced by an amount δR. The permissible amount of displacement dR depends on preloading value v. The calculation of the optimum preloading value should take into account factors of service life, running behavior and guiding stability.

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
From the rules of rolling friction, it is known that ball diameter k affects the degree of friction, i.e. a larger ball rolls more smoothly than a smaller one.

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.

Rotary Stroke Bearings
Coefficient of friction µ
EN

The coefficients of friction µ apply for the start-up run and movement alike.

Influencing variables:

  • Surface condition of the rolling elements
  • Degree of preloading and load
  • Number of balls
  • Friction of cage

The MarMotion high-precision rotary stroke bearings run free of stick-slip. The following coefficients of friction apply to radial load:

high µ = 0.001–0.002
medium µ = 0.003–0.004
low  µ = 0.005–0.008

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.

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