How do I calculate the required load capacity and safety factor?

The basis of every safe and reliable drive design.

Correctly determining the load capacity and applying appropriate safety factors are the first and most important steps when selecting a screw jack. Underestimating the load or neglecting safety reserves can lead to overload, premature wear and, in the worst case, failure of the system.

Step 1: Determining the maximum operating load

Identify and quantify all forces acting on the gearbox. Distinguish between:

• Static load: Force at standstill (holding force).
• Dynamic load: Force required to move the load – including friction, accelerations and, where applicable, process forces.

Notes: In many applications, the dynamic load is higher than the static load. For multiple gearboxes acting on one load, the load distribution must be checked; in case of doubt, design on the basis of the most highly loaded axis.

Step 2: Applying the safety factors

Safety factors create reserves for scatter, environmental influences and short-term load peaks. Recommendation (minimum values):

• Static load: +30% safety margin
• Dynamic load: +10% safety margin

The selected screw jack must have static and dynamic load capacities that are at least equal to the design loads calculated in this way.

Example

• Holding: 7.0 kN
• Moving (incl. friction): 7.5 kN
• Design static: 7,0 kN × 1,30 = 9,1 kN
• Design dynamic: 7,5 kN × 1,10 = 8,25 kN
• Selection: Select a screw jack with permissible static load capacity ≥ 9.1 kN and permissible dynamic load capacity ≥ 8.25 kN. A gearbox with a rated load of 10 kN is suitable in this example.

Step 3: Taking buckling load into account

For long screws under compressive load, the buckling load is often the limiting factor – not the rated load of the gearbox. Therefore, always carry out a separate buckling check for the screw. Recommendation: Safety factor against buckling 3–6, depending on end conditions and application.

Short conclusion

1. Fully determine static and dynamic loads.
2. Add safety margins (+30% static, +10% dynamic).
3. Select the gearbox on the basis of the design loads.
4. Perform a separate buckling verification for the screw.

This ensures a robust, safe and long-lasting design – without unnecessary complexity.