FAQs for worm gear screw jacks

WHAT IS THE POWER RANGE OF WORM GEAR SCREW JACKS?

Worm gear screw jacks are available for nominal loads between 0.5 and 100 tons. At the lower end, this realistically means loads of only 0.1 tons (1 kN). In the upper range, 250 tons are possible on request.

The linear speeds can be between 0.3 and 1,000 mm/s, and variable speed drives can reduce these values even further. Strokes of up to 6 m are possible.

HOW DO I CHOOSE BETWEEN THE VERSIONS WITH TRAVELLING SCREW AND WITH TRAVELLING NUT? 

The performance of these two options is the same, so the choice depends mainly on the installation requirements. In the versions with a travelling screw, the screw is guided through the housing of the screw jack. This means that the screw always remains under the load, but space is required on the other side of the housing. For versions with a travelling nut, the screw and the housing are in a fixed axial position, with the nut moving up and down on the screw.

Note that mounting can be done in any orientation. In fact, tensile forces may be better than compressive forces as there is no tendency to buckle. Lateral loads should be avoided.

WHAT IS THE DIFFERENCE IN PERFORMANCE BETWEEN TRAPEZOIDAL AND BALL SCREW JACKS? 

Ball screw jacks are more efficient than ACME screw models. This enables higher duty cycles, higher speeds or more compact dimensions for a given force. The energy requirement of ball screw jacks is lower, but their acquisition costs are higher. The choice between the two types depends on the required duty cycle, loads and speeds. The downloadable quick selector provides a simple overview of their performance.

HOW IS THE LINEAR SPEED VARIED? 

Worm gear screw jacks work on the principle of a worm gear and typically 3 or 4 worm gear ratios are used to produce a range of linear speeds. Another variable is the number of starts of the screw, which can be between 1 and 4. A higher number of starts results in a higher linear speed, but the load capacity may be reduced.

Ball screw jacks have a similar worm gear with variable ratios and a ball screw with nut that replaces the ACME screw. The linear speeds can be varied by the pitch of the ball screw.

High-speed ball screw jacks use bevel gears instead of worm gears, resulting in flat ratios, higher speeds and higher efficiencies.

The input speeds of screw jacks can be as low as those wound by hand or as high as those of motors (usually 4-pole 1,500 rpm, but 2-pole 3,000 rpm is also possible).

WHICH WORK CYCLES ARE POSSIBLE? 

Typical maximum duty cycles for standard worm gear screw jacks are between 20 and 40%. For shorter running times over a period of 10 minutes, the duty cycle is 30-40%, depending on the model. For longer running times, a period of 60 minutes is used as a basis and the duty cycle is 20-30%. If higher duty cycles are required, there are two options:

• Oversizing the worm gear screw jack so that the required power compared to the maximum power results in a factor that can increase the permissible duty cycle.
• You choose a worm gear screw jack with a ball screw, which is available in the range from 5 kN to 400 kN. These have a higher efficiency and duty cycles of 70-100 % are possible.

WHAT ARE THE LIMITING FACTORS FOR PERFORMANCE?

The stroke length of 6 m is limited by the production facilities.

Further limits result from the buckling load and the critical speed of the ACME screw.

The buckling load limit in turn depends on the type of screw guide. Unguided screws have relatively low limits for force and stroke. Buckling limits are generally not relevant for fully guided screws. There are no buckling limits for tensile loads.

The critical speed limits also depend on the screw fastening and the stroke length. The critical speed is not relevant for screws with a bearing-mounted end and strokes of up to 1.5 m.

WHICH MOTORS CAN BE USED? 

Worm gear screw jacks can be motorized by connecting electric motors via a B14 flange or a B5 bell housing with coupling. Usually 3-phase AC motors are used, but 1-phase and DC motors are also possible. Servomotors can be integrated and their high speeds and power are useful for ball screw jacks. Brake motors are occasionally needed when a quick stop is required or when the screw jack is not self-locking.

HOW SHOULD THE HUB BE CONTROLLED? 

Screw jacks should not be driven to their mechanical limits, especially models with higher speeds. A simple stroke control can be carried out with limit switches. These can be mounted externally or attached to the cover tube on models with moving spindles.

Alternatively, the stroke position can also be controlled via a motor encoder feedback to a suitable drive or control system. Limit switches are still recommended to ensure safety.

WHAT OTHER OPTIONS ARE REQUIRED?

There are many modular options for screw jacks; sometimes they depend on the choice of design between moving screw and moving nut.

For travelling screw models, a protective tube can be used to cover the non-active side of the screw. This also allows the installation of limit switches or an anti-rotation device.

The anti-rotation device is used in cases where the load to be moved can rotate. In such cases, the spindle and load rotate without axial movement, unless the anti-rotation device is selected, which consists of a keyway in the protective tube. For applications that require a high degree of positioning accuracy, both worm gear screw jacks can be fitted with adjustable play.

The pivot mounting of the housing allows the axis to swivel so that the force can be transmitted without side loading.

WHAT ABOUT DIFFICULT ENVIRONMENTS?

Although the housings are fully encapsulated and maintenance-free, the spindles must be lubricated regularly in accordance with the maintenance manual. The screws must be kept adequately free from contamination, for which purpose bellows can be used. Under certain conditions, it may be advisable to opt for a stainless steel screw (AISI 303, 304 or 316).

CAN A LIFTING SYSTEM BE PROVIDED? 

Several worm gear screw jacks are often required to lift a load synchronously. They can be mounted in series or in a T, H or U configuration. In either case, a single motor or geared motor is used and the connection is made via intermediate shaft couplings and bevel gearboxes as required.

Such systems are readily possible, but care must be taken, particularly with regard to the dimensions. Clutch speeds may be limited, so a gearbox is often used to reduce the engine speed. Ideally, metal disk clutches are chosen as they are torsionally stiff. In some cases, claw clutches with high rigidity can also be used at a lower cost.

HOW HIGH IS THE EFFICIENCY OF WORM GEAR SCREW JACKS? 

The efficiency of worm gear screw jacks varies from model to model and depends on the worm gear ratio and the ACME thread pitch. Details are given in the catalog. As a rule, an ACME worm gear screw jack has an efficiency of around 30 %.

Ball screw jacks are usually twice as efficient. The SJ BS models have an efficiency of around 65 %, the MA BS models of around 70 % and the HS high-speed models can achieve an efficiency of around 90 %.