Selecting lead screws
How do you choose the right lead screw for your application? We ask the experts at Automotion Components for advice.
From linear guides to actuation, the lead screw is a staple of the machine building world. But selecting the right component for your design is no trivial task, with many factors coming into play. Here, we look at some of those design issues, and how they might lead you in particular directions.
Perhaps the first consideration is the working environment, as this will play a key role in defining the material you will choose. For environments where there are no particular corrosive or oxidising agents, steel lead screws can be used. Where these conditions are not met, however - for example in areas with a relative humidity of 70-80% and above, or where the lead screw will be immersed in water (including sea water), or where the screw will operate in the presence of corrosive agents such as chlorides - then stainless steel is a better choice of material.
Stainless steel is also the most appropriate material for the food and pharmaceutical industries, perhaps with stainless steel or bronze nuts. And, when used with plastic nuts, stainless steel is ideal for applications where the lead screw cannot be accessed for lubrication. It also meets the needs of high temperature applications (above 200°C) because stainless steel has a structure that is better suited to higher temperatures.
Understanding backdrive
Another important consideration is backdrive. Irreversibility defines how much the nut can backdrive down the leadscrew. If it cannot at all, then the screw is said to be irreversible. This is especially important if the lead screw and nut are being used in a vertical application, where you almost certainly wouldn't want any backdrive.
Lead screws installed with an angle of less than 2° 30' are always completely irreversible and cannot backdrive. Lead screws with a lead angle above 5° but less than 6° still have a good degree of irreversibility but may in some instances exhibit some backdrive. Lead screws installed at an angle above 6° have zero irreversibility, so the nut may backdrive down the lead screw with little or no load applied. If backdrive is going to be a problem in an application, then some remedial action will need to be taken. A finer lead screw pitch will help, but there comes a point where a locking mechanism will be needed.
What about the choice of nut? Bronze and stainless steel nuts are resistant to oxidising agents that occur in various applications. In applications where added lubricant is not allowed, self-lubricating plastic nuts are ideal. However, plastic nuts are impacted by specific working conditions, with restrictions on the working temperature and problems with moisture absorption.
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