Page 15 of catalogue Ground ball screws of SKF Linearsysteme GmbH

ensures conditions of no play is, as seen above, equal to FA / 2,83. Once the ball screw has been dimensioned with the calculated required rigidity, a further increase n the preload does not lead to any very noticeable increase in rigidity (➔ fig. 7) but tends to reduce ball screw life due to the increase in the operating torque and in temperature. Each time the temperature increases by one degree above 20 °C, there is an approx. 0,01 mm elongation per degree and per meter in the steel used to construct the precision ball screw.

Preloading systems In addition to the above-mentioned system, in which two preloaded Nuts are used, the single preloaded nut system can be applied by using larger-sized balls (with four contact points) or with a shift in the lead of the nut tracks. Permissible deviations for the preload torque (ISO/DIS 3408-3 Draft Standard) table 8 gives the maximum permissible tolerance values ± ΔTpp in % in relation to the nominal torque Tpo; the effective values Tpa and ± ΔTpa measured with the procedure outlined in the paragraph above must be within this range.

Materials and thermal expansions
SKF BSS ball screw shafts are made of particularly impuretyfree steels, able to withstand the heat treatments applied without cracking or uncontrolled deformations. The track-ball contact area is surfacehardened by applying strictly controlled induction hardening procedures for the screw shafts and casehardening procedures for the nuts followed by deep freeze treatment (for the residual austenite) and soft tempering. Constant hardening thicknesses of ≥ 2 mm are thus obtained with hardness values of 59 ... 62 HRC. The ends of the Screws are usually hardened and tempered (R = 80 ... 90 daN/mm2). The thermal expansion coefficient of the screw is K a = 12 10 – 6/degree; the resulting axial elongation at a thermal gradient of Δθ [°C] is therefore: Δl = K a⋅Δθ⋅L [mm] This should be taken into account when selecting the correct preload and lead compensation in order to obtain optimal working conditions.

3

Fig. 7

F

D1

Table 8 Tp0 [Nm] ΔTpp (% of Tp0 ) L u /d 0 < 40; L u < 4000 mm L u /d 0 < 60; L u < 4000 mm L u > 4000 mm

from
0,2 0,4 0,6 1 2,5 6,3

to
0,4 0,6 1 2,5 6,3 10

ISO 1
35 25 25 20 15 –

ISO 3
40 40 30 25 20 15

ISO 5
50 40 35 30 25 20

ISO 7
– – 40 35 30 30

ISO 1
40 33 30 25 20 –

ISO 3
50 40 35 30 25 20

ISO 5
60 45 40 35 30 25

ISO 7
– – 45 40 35 35

ISO 1
– – – – – –

ISO 3
– – 40 35 30 25

ISO 5
– – 45 40 35 30

ISO 7
– – 50 45 40 35

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