Frequently Asked Questions - Screw Jacks
What is the performance range of screw jacks?
Screw jacks are available for rated loads between 0.5 and 100 tonnes. At the bottom end this realistically means loads as low as 0.1 tonne (1kN). At the top end 250 tonnes is possible on request.
Linear speeds can be from 0.3 to 300 mm/s, and variable speed drives can further reduce these figures. Strokes up to 6m are possible.
How do I choose between travelling screw and travelling nut designs?
Performance of these two options is the same so the choice depends mainly on the mounting requirements. Travelling screw versions have the screw passing through the housing of the jack. Thus the screw remains below the load at all times, but space is needed on the other side of the housing. Travelling nut designs have the screw and housing in a fixed axial position with the nut passing up and down the screw.
Note that mounting can be in any orientation. In fact pull forces can be better than push forces because there is no tendency for buckling. Side loads should be avoided.
How is the linear speed varied?
Screw jacks operate with the principle of a worm gearbox and typically 3 or 4 worm gear ratios are used to create a range of linear speeds. An additional variable is the number of starts of the screw, between 1 and 4 starts are possible. Increasing the number of starts gives increased linear speed but load capacity may reduce.
Screw jack input speeds can be as low as ‘hand winding’ or as high as motor speeds (usually 4 pole 1500 r/min but 2 pole 3000 r/min can be possible)
What duty cycles are possible?
Typical maximum duty cycles are between 20 and 40%. When calculated with shorter running times over a 10 minute period duty cycles are 30-40% dependent on model. For longer running times a 60 minute period is used and duty cycles are 20-30%. Where higher duty cycles are needed, there are two options.
-oversize the screw jack so that the power required compared to the maximum power gives a factor that can increase the permissible duty.
- select a ball screw jack available in the range 5kN to 350kN. These have higher efficiency and duty cycles of 70-100% are possible.
See catalogue pages 8-10.
What are limiting factors to the performance?
The stroke length of 6m is limited by production facilities.
Further limits relate to buckling and critical speed of the acme screw.
The buckling load limit depends in turn on the way the screw is guided. Unguided screws have relatively low limits for the force and the stroke. If the screw is fully guided buckling limits are generally not relevant. See catalogue pages 13-15. Where the load is in tension, there are no buckling limits.
Critical speed limits also depend on the screw fixing and the stroke length. For screws with a supported end and strokes up to 1.5m, the critical speed will not be relevant. See catalogue pages 16-17.
What motors can be used?
Screw jacks can be motorised by connecting electric motors through a B14 flange or a B5 bell housing with coupling. Commonly motors are AC 3 phase although 1 phase and DC motors are also possible. Technically servo motors can also be used, but their high dynamic performance is rarely required for screw jacks.
Brake motors are occassionally needed where fast stopping is required or if the screw jack is not self-locking. See catalogue page 12.
How should the stroke be controlled?
Screw jacks should not be driven into their mechanical limits, particularly models with higher speeds. Simple stroke control can be provided with limit switches. These can be mounted externally, or in the case of travelling screw models they can be fitted to the cover tube, catalogue pages 45 and 79.
Alternatively control of the stroke position can be achieved by motor encoder feedback to a suitable drive or controller. Limit switches are still recommended to provide a safety back-up.
What other option are needed?
There are many modular options for screw jacks; sometimes they depend on the choice of design between travelling screw and travelling nut.
A Protective tube can be used to cover the non-active side of the screw with travelling screw models. This also allows the mounting of limit switches or an anti turn device. Catalogue pages 38 and 72.
The anti-turn option is used in cases where the load to be moved is capable of rotation. In such cases the screw and load will spin without axial movement unless the anti-turn device, which is a keyway in the protective tube, is chosen.
Bellows can provided for both designs when used in particularly difficult environments. Catalogue pages 40 and 74.
Higher levels of safety can be integrated. For both designs a safety nut is available. This is a back-up feature that prevents the load dropping in an uncontrolled manner in the event of thread breakage due to overload or wear. The safety nut works in one direction only. Catalogue pages 42 and 76. Also the wear can be monitored using a thread wear option which gives a limit switch output when a set level is exceeded, catalogue pages 43 and 77.
For applications that require a high level of positioning accuracy, both designs of screw jack can be provided with an adjustable backlash feature, catalogue page 46.
Trunnion mounting of the housing allows the axis to pivot delivering the force without side loads, catalogue pages 39 and 73. Such side loads should be avoided, but where some are inevitable a bronze guide in the housing is recommended, catalogue page 37.
What about difficult environments?
Whilst the housings are fully enclosed and maintenance free, the acme screws must be periodically greased in line with the maintenance manual. Screws must be kept reasonable clean from contaminant and bellows can be fitted to achieve that, catalogue pages 40 and 74. In some conditions it may be sensible to opt for a stainless steel screw to AISI 303, 304 or 316.
Can a lifting system be provided?
Frequently several screw jacks are required to lift a load in synchronisation. They can be mounted in line or in a T, H or U configuration, see catalogue page 94. In each case as single motor or geared motor is used and connection is made by spacer shaft couplings and bevel gearboxes as required.
Such systems are readily possible but care is needed particularly on the dimensional layout. Coupling rotation speeds may be limited and hence a gearbox is often used to reduce the motor speed. Ideally metal disc shaft couplings are selected because they are torsionally rigid. In some cases jaw couplings with high stiffness may be acceptable at a lower cost.