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 (1 kN). At the top end 250 tonnes is possible on request.
Linear speeds can be from 0.3 to 1,000 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.
WHAT IS THE PERFORMANCE DIFFERENCE BETWEEN ACME AND BALL SCREW JACKS?
Ball screw jacks are more efficient than ACME screw models. This allows higher duty cycles, higher speeds or more compact dimensions for a given force. Energy requirements for ball screw jacks will be lower but their initial cost is higher. The choice between the two types depends on the required duty cycle, loads and speeds. The downloadable quick selector gives an easy summary of their performance.
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.
Ball screw jacks feature a similar worm gearbox with variable ratios and a ball screw & nut replacing the ACME screw. Linear speeds can be varied by the pitch of the ball screw.
High speed ball screw jacks use bevel instead of worm gears leading to shallow ratios, higher speeds and higher efficiencies.
Screw jack input speeds can be as low as 'hand winding' or as high as motor speeds (usually 4 pole 1,500 r/min but 2 pole 3,000 r/min can be possible).
WHAT DUTY CYCLES ARE POSSIBLE?
Typical maximum duty cycles for standard screw jacks 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 5 kN to 400 kN. These have higher efficiency and duty cycles of 70-100% are possible.
WHAT ARE LIMITING FACTORS TO THE PERFORMANCE?
The stroke length of 6 m 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. 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 1.5 m, the critical speed will not be relevant.
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. Servo motors can be integrated and their high speeds and performance are useful with ball screw jacks. Brake motors are occasionally needed where fast stopping is required or if the screw jack is not self-locking.
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.
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.
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 provide for both designs when used in particularly difficult environments.
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. Also the wear can be monitored using a thread wear option which gives a limit switch output when a set level is exceeded.
For applications that require a high level of positioning accuracy, both designs of screw jack can be provided with an adjustable backlash feature.
Trunnion mounting of the housing allows the axis to pivot delivering the force without side loads.
WHAT ABOUT DIFFICULT ENVIRONMENTS?
Whilst the housings are fully enclosed and maintenance free, the 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. 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. 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.
WHAT IS THE EFFICIENCY OF SCREW JACKS?
The efficiency of screw jacks varies from model to model and depends on the worm ratio and the ACME thread pitch. Details are given in the catalogue. Typically, an ACME thread screw jack has efficiency at around 30%.
Ball screw jacks have efficiencies that are typically two times higher. SJ BS models are around 65%, MA BS at about 70%, and the HS high speed models can be around 90%.