Automotion Components Lead Screws
What are lead screws?
Lead screws are important mechanical components that translate rotational motion (for instance, from a motor or handwheel) into linear motion. The 'lead' is the linear travel achieved by one full (360o) rotation of the lead nut. We can provide special lengths and machine the ends of the lead screws to your requirements on request.
When choosing a lead screw, you must know the distance it must travel, the load it can support, the amount of use you expect it to experience, and more. Other factors you may wish to consider are detailed on this page - for example, there can be compressive (buckling) forces depending on how you load the lead screw. On this page, we have collated all the technical information for lead screws that should help you choose the correct lead screw system for your intended application.
Product videos
I: Introduction to lead screws
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II: Application and setup of lead screws
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III: Selection of guides and lead screws
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Product Selection
The choice between different types of lead screws and lead screw nuts available is generally made after taking into account the following factors.
Choice of lead screw
Working environment
For work environments where there are no particular corrosive or oxidising agents our steel (L1320 and L1321) lead screws can be used.
Where these conditions are not met, we recommend using our stainless steel screws (L1322 and L1323) which are particularly suitable in the following cases:
- With a relative humidity of 70-80% and above.
- Immersed in water (including sea water).
- In the presence of particular corrosive agents such as chlorides. In case of highly corrosive agents please contact our Technical Department.
- In the food industry or pharmaceutical industry, where they are used with stainless steel or bronze nuts.
- Where the lead screws cannot be reached for lubrication. In particular, for lubricating “maintenance free” fi ttings they are coupled with plastic nuts.
- Where working temperatures are relatively high (above 200°C) - because stainless steel has a structure that is more suited to higher temperatures.
Backdrive
Irreversibility defi nes how much the nut can “backdrive” down the lead screw. If a nut cannot backdrive down the screw then it is irreversible. This is especially important if the lead screw and nut are being used in a vertical application i.e. in this situation no backdrive is normally acceptable.
Lead screws with a lead angle of <2° 30’ are completely irreversible i.e., they cannot backdrive. Lead screws with a lead angle >5° but <6° still have a good degree of irreversibility and may in some instances exhibit some backdrive.
Lead screws with a lead angle >6° have zero irreversibility, therefore the nut may backdrive down the lead screw with little or no load applied.
This is important to know in vertical applications.
Choice of lead screw nut
Working environment
Our bronze and stainless steel 303 lead screw nuts, are resistant to standard oxidizing agents that occur in various applications.
Where corrosive agents are present, please contact our Technical Department for advice.
In applications where the presence of added lubricant (grease or oil) is not allowed we recommend the use of self-lubricating plastic nuts.
The use of plastics can however be limited by the specific working conditions, therefore please consult our Technical Department should you wish. This is because plastics have excellent self-lubrication features, but at the same time have restrictions on the working temperature or moisture absorption problems, (as well as some mechanical properties that may not be suitable for the intended use).
Single start or double start
Pitch
The axial distance between threads. Pitch is equal to the lead in a single start screw.
Lead
The axial distance the nut advances in one revolution of the screw. The lead is equal to the pitch times the number of starts.
Lead = Pitch x No. of starts
For example: A 10mm diameter lead screw has a pitch of 2mm. On a single start lead screw the lead is also 2mm. On a twin start lead screw the lead is 4mm
Screw starts
The number of independent threads on the screw shaft, example one or two.
General Guidance
Compact Rail
Advantages
Easy to install. Allows for some misalignment at installation. Compact system, small footprint compared to other rail systems. Preload adjustable by hand.
Disadvantages
Although it can take very large loads it cannot take anywhere near as much load as the linear guideways rail system.
X Rail
Advantages
Cheaper alternative to the compact rail. Use of T and U rails allows for misalignment at installation. Preload adjustable by hand.
Available in AISI 316L stainless steel suitable for use in applications requiring a high level of corrosion resistance, including sea water.
Disadvantages
Cannot take as much load as other systems. Not suitable for moment loads.
Linear guideways
Advantages
Can take extremely high loads including moment loads. Very smooth in operation.
Disadvantages
Must be aligned very accurately which costs time and money preparing the mounting surface properly.
Shafts + bearings
Advantages
Available in a range of materials and sizes. A length of shaft bar is typically cheaper than the cost of an equivalent size precision linear rail.
Disadvantages
A larger diameter shaft would be required when there are long lengths and high loads involved compared with the size of an equivalent shaft support rail you would need. This is because they would only be supported at the ends and the shaft ends could bend in the middle if the diameter was too small.
Shaft support rail
Advantages
Shaft support rail systems have a shaft support along the full length so are less likely to flex.
Disadvantages
Similar to linear guideways, must be aligned very accurately otherwise any misalignment may cause the system to ‘snatch’.
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