Dimensioning Screw Jacks

What calculations should be done before purchasing a screw jack?

1. Motor power

Motor power is crucial to ensure that the lifting element can lift or move the required load. The motor's power should be sufficient to achieve the desired torque and speed, depending on the size and weight of the load to be moved.

1.1 Determining motor power

1. Dynamic lifting force Fdyn = m * g in kN (g = 9.81 m/s² where 10 m/s² is assumed)
2. Required drive speed

1. Drive torque of the system

1. From the drive torque and the required drive speed, the formula for determining the motor power PM is derived.

2. Speed

The speed of a screw jack refers to the rate at which the linear motion of the screw jack occurs. It is important to understand how the speed works in a screw jack, as it has a significant impact on the performance and application of the screw jack.

The allowable spindle speed is then determined by:

The allowable speed is typically calculated without the nut. As a result, the unsupported spindle length changes permanently, and a higher speed may be permitted under certain circumstances.

3. Buckling

Buckling refers to the deformation or bending of lifting system components under the influence of compressive forces or loads. Buckling can occur when an elongated component, such as a spindle or a push rod, is subjected to a compressive load that exceeds a critical threshold.

Buckling can be avoided by selecting components with sufficient stiffness and limiting the load to values below the buckling load.

For a rough preliminary dimensioning, the following formula is sufficient:

3.1 Euler 1

3.2 Euler 2

3.3 Euler 3

To ensure that the components of the screw jack withstand the loads, buckling can be analyzed through more detailed calculations, which are shown below:

The limiting slenderness ratio indicates how slender or relatively thin a spindle is in relation to its length. A lower value means the spindle is relatively thick compared to its length, while a higher value indicates that the spindle is more slender in relation to its length.

A too high limiting slenderness ratio can lead to instability, causing vibrations or oscillations in the spindle. Conversely, a low limiting slenderness ratio may mean that the spindle is unnecessarily large and heavy.

If the slenderness ratio λ of the spindle is < λ0, the buckling calculation is performed according to Tetmajer (inelastic).

The slenderness ratio can be calculated using the following formula:

3.4 Determination of buckling stress according to Tetmajer

3.5 Determination of buckling stress according to Euler

4. Bending critical speed

Long, rapidly rotating spindles can begin to oscillate under both compressive and tensile loads. This calculation is considered in the critical speed. The critical speed must be taken into account only for the traveling nut version, as spindle rotation occurs only in this case. Factors to consider include the diameter and length of the spindle, as well as its bearing.

5. Lead angle

The lead angle is an important parameter in the dimensioning of screw jacks. It refers to the angle at which the threaded spindle or worm shaft of a screw jack is inclined. The lead angle affects how the screw jack converts linear motion into rotational movement and vice versa.

A larger lead angle results in more linear movement per revolution of the spindle or worm. A smaller lead angle leads to higher speed but lower force transmission, whereas a larger lead angle results in lower speed but higher force transmission.

Here is how to calculate the lead angle:

When dimensioning lifting platforms with threaded spindles as the drive mechanism, the following rules apply for the lead angle of the screw as well as any potential self-locking of the thread: