Page 9 of catalogue Ground ball screws of SKF Linearsysteme GmbH

Assembly procedure

Note.: Ground ball Screws are precision Components and should be handled with care to avoid shocks. When stored out of the shipping crate they must lie on wooden or plastic vee blocks and should not be allowed to sag. Screw assemblies are shipped, wrapped in a heavy gauge plastic tube which protects them from foreign material and possible pollution. They should stay wrapped until they are used.

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Radial and moment loads
Any radial or moment load on the nut will overload some of the contact surfaces, thus significantly reducing its life.

Designing the screw shaft ends
Generally speaking, when the ends of the screw shaft are specified by the customer’s engineering personnel, it is their responsability to check the strength of these ends. However, we offer in pages 16 and 17 of this catalogue, a choice of standard machined ends. As far as possible, we recommend their use. Whatever your choice may be, please keep in mind that no dimension on the shaft ends can exceed do (otherwise traces of the root of thread will appear or the shaft must be made by joining 2 pieces). A minimum shoulder should be sufficient to maintain the internal bearing.

Operating temperature
Screws made from standard steel and operating under normal loads can sustain temperatures in the range –10 °C ÷ + 70 °C. Above 70 °C, materials adapted to the temperature of the application should be selected. Consult SKF for advice.

Alignment
SKF linear guidance components should be used to ensure correct alignment and avoid non-axial loading. The parallelism of the screw shaft with the guiding devices must be checked. If external linear guidance prove impractical, we suggest mounting the nut on trunnions or gimbals and the screw shaft in selfaligning bearings. Mounting the screw in tension helps align it properly and eliminates bucking.

Starting-up the screw
After the assembly has been cleaned, mounted and lubricated, it is recommended that the nut is allowed to make several full strokes at low speed; to check the proper positioning of the Limit switches or reversing mechanism before applying the full load and the full speed.

Lubrication
Good lubrication is essential for the proper functioning of the screw and for its long term reliability(1). Before shipping, the screw is coated with a protective fluid that dries to a film. This protective film is not a lubricant. Depending on the selected lubricant, it may be necessary to remove this film before applying the lubricant (there may be a risk of non-compatibility). If this operation is performed in a potentially polluted atmosphere it is highly recommended to proceed with a thorough cleaning of the assembly.

Note: Operating at high temperature will lower the hardness of the steel, alter the accuracy of the thread and may increase the oxidability of the materials.

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