here are thousands of off-the-shelf lead screws and related components to choose from, and while experienced motion
engineers and automated selection tools can get you close to an ideal solution, optimisation will almost always require a degree of customisation. Understanding the wide range of available options will help you determine the need for customisation in your applications. But let’s start by explaining their design. Lead screws consist of a threaded shaft and a nut that carries the load along the shaft or guides the shaft in moving the load, with energy derived from a servo or stepper motor.
Customisation must always begin with an appreciation of the end-user’s needs, which will translate into specific design requirements and, ultimately, the motion solution. Lead screw shafts integrated with servo or stepper motors offer what may be the widest range of customisation options. Common candidates for customisation:
• Screw shafts, such as changing pitch or leads, length, stroke and end journaling.
• Nuts, such as carriage shape and size, flanges and press fitting.
• Materials, such as changing stainless steel bearings, or aluminium or plastic housings; nuts or bushings made in a special metal, plastic, composite or ceramic materials.
• Surface treatments, such as paint quality or colour, and thickness of anodized aluminium parts.
• Mounting, such as orientation brackets, along with various couplings or angles.
• Connections, such as tooling. • Keyways, such as flats to secure the screw to a motor or driven component; bore sizes.
• Integrations, such as hybrid systems combining screw, nut, gearing, rail support and stepper motor.
• New designs, such as adapting existing products for new functionality or designing a new product or system from scratch.
• Services, such as lubrication, stocking, transportation, training, repair and maintenance.
Among the most common customisation objectives are improving performance, optimising space, extending durability, resisting environmental threats, complying
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with regulations, and conversion from pneumatic cylinders. But while using configuration options and
online tools will bring you close to an effective motion solution, it usually takes a degree of customisation to reach your desired performance, physical attributes, regulatory compliance and cost-saving objectives. Custom screw machining to maintain
consistent diameters across the length of the screw can smooth operations and reduce axial play. Adjusting pitch or leads can improve speed by increasing the number of revolutions of the screw. Additional grinding, lapping and position error adjustment (PEV) can also improve accuracy and general performance. Shortening screw lengths to enable handling of higher loads by distributing them over less space can improve load capacity. This lowers the bending moment and, with it, the potential for buckling, improving the screw’s overall load-bearing capacity and strength. Fitting an encoder to the back of the motor
improves control by enabling system feedback (Figure 1). Getting the right feedback scenario could require customising the gear ratio, motor resolution and lead screw pitch. Mechanical, contact or optical limit switches can help control the movement of the nut across the screw. Backlash nuts contribute to accuracy by eliminating play, and rethreading the screw can enable bidirectional movement.
Most applications benefit from more efficient use of space. Trimming the screw to the shortest length needed to deliver the required stroke will contribute to space efficiency, as can modifying the housing. Increasing thread pitch to a higher lead angle or a more compact thread form can also help make more efficient use of shorter screws. To reduce nut size, choose materials with a higher strength-to-weight ratio, which can deliver a slight advantage in other areas. Precision CNC machining can also optimise trim sizes, adding precision as well as compactness. For weight reduction, grinding the screw
to a narrower diameter can be beneficial, but customising material selection will have more impact. Most lead screw vendors offer standard material options, covering different grades of quality, but when lightness is a key part of the product function, it may require non-standard materials such as plastic gears. Customising the material is also the primary method of reducing noise. In addition to managing size and shape, the designer controls durability, minimising operating time and related costs. Material selection comes into play here, where highly resistant materials such as stainless steel or carbon steel can resist wear, while also adding strength. Applying custom dry coatings such as PDFE can improve durability as well, but the greatest material impact on durability is self- lubricating bearings. Custom material selection and coatings are also important for extending the life of equipment in extreme temperatures or other challenging conditions.
Figure 1: Customising a lead
screw motor with an encoder can benefit linear motion applications
Cost reduction is arguably the most common objective among lead screw customisations. Performance improvements can increase operating efficiency and reduce material waste. Switching from grinding to a cold-rolling process, for example, can reduce production costs. Altering the thread design to match the specific application requirements can enhance
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