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
- Fully determine static and dynamic loads.
- Add safety margins (+30% static, +10% dynamic).
- Select the gearbox on the basis of the design loads.
- Perform a separate buckling verification for the screw.
This ensures a robust, safe and long-lasting design – without unnecessary complexity.
