the Thrust indicates the force with which a window actuator can push the sash open. It is given in newtons (N) . The opposite direction, i.e. pulling closed, is called pulling force. With many actuators both values are the same, with some they differ.
What does the required thrust depend on?
- Sash weight: heavier sashes need more force
- Mounting position: a roof window or rooflight is lifted by the actuator against gravity, a tilt window in the façade less so
- Opening width and lever arm: where the actuator sits on the sash
- Wind and snow: external loads, especially on roof surfaces
- Seals: the actuator must also pull the sash tightly closed
Rule of thumb: better plan a reserve
An actuator that constantly works at its limit has a shorter life. So plan a force reserve. How to estimate the force more precisely is explained in the guide Technical design of window drives.
Thrust ratings at Vent Systems
- VS-KA chain drive: 250 to 1500 N
- VS-KA 2500: 2500 N for heavy sashes
- VS-LA linear actuator: 600 to 5000 N, e.g. for rooflights
- VS-SA 2500 spindle drive: 2500 N
Calculation example: horizontal flap
A horizontal flap weighs 30 kg, i.e. around 300 N. If it is hinged on one side and the actuator acts on the opposite edge, it has to lift about half the weight: around 150 N. On top of that come friction, seal pressure and wind and snow load. As a rule of thumb, an actuator with about double the reserve makes sense here, i.e. from around 300 N. With snow load or large areas, correspondingly more.
Common mistakes
- Only the glass calculated, not the whole sash with frame and fittings
- Wind load forgotten for smoke vents – they must also open against the wind
- Only one actuator planned for wide sashes (see tandem operation)
Not sure about the force? The actuator finder helps you choose.
Frequently asked questions
What is the difference between thrust and pulling force?
Thrust acts when opening, pulling force when closing. For windows that open outwards, thrust is usually decisive.
Is more thrust always better?
Not necessarily. Too much force strains the frame and fixings and increases the risk at the closing edges. Choose sufficient reserve, but suited to the window.
Read more
- Guide: calculating thrust with worked examples
- Glossary: stroke length
- Glossary: tandem operation – two actuators on one sash
Not sure which actuator fits? Call us: 040 50694631 (Mon–Fri 8 am–5 pm) or use the actuator finder.