The new pre-load screw nut

The question of clearance is often asked in the case of plastic lead screw nuts. This describes the lost motion, i.e. the "dead travel" on the lead screw. Or in other words the gap between the thread flanks of the lead screw and the lead screw nut. The term clearance is often used in general. In this context, the correct technical term is axial clearance. It outlines the movement of the lead screw nut in the axial direction without movement of the lead screw. Lead screw drives require a minimum clearance to maintain their function. If the axial clearance is too small, the torque required to move the lead screw nut also increases.

In addition to the axial clearance caused by the manufacturing tolerances, the application-specific influences must also be taken into account. In addition to thermal and hygroscopic environmental influences, the minimum clearance to be considered in the application must also take into account the friction heat generated by the application. This plays a decisive role in connection with the expansion behaviour of the polymer material used. This means that the axial clearance in the system can change with temperature fluctuations.

In addition to the right choice of material, a preloading mechanism has proven to be an effective measure to reduce unwanted clearance. For our dryspin® high helix threads and trapezoidal threads there are four versions with pretension: Zero-Backlash, Preload, Low-Clearance and AB lead screw nut.The working principle of the pre-load lead screw nut
PL lead screw nuts reduce the axial clearance in a lead screw drive by preloading two lead screw parts with a compression spring. The spring pushes the two halves of the lead screw nuts apart axially in the direction of the lead screw flanks. As long as the axial load in the lead screw drive is lower than the spring pre-load, the entire lead screw drive has a minimised backlash. This also permanently reduces clearance caused by wear.

Lubrication-free operation and high precision are very important, especially in medical technology. Due to the low-clearance and cost-effective function of the PL lead screw nut, numerous advantages can be achieved for both horizontal and vertical movements:

• Clean operation due to lubrication-free and maintenance-free properties
• Minimum axial backlash without readjustment
• Precise positioning and infeed movements
• Smoother operation and fewer vibrations thanks to the combination of the dryspin® thread profile and the pretension of the lead screw nut

The price and technology advantages can be combined optimally in both pipette head adjustments and sample collection. For applications with high loads, high temperatures or in heavily polluted environments, a standard lead screw nut should always be used. The reason for this is that all four principles are cancelled out by high loads. For this reason, the maximum static axial loads are specified separately for each lead screw nut model.