Everything you need to know about lead screws

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This article from Abssac explains lead screws and nuts, which are still an excellent and economical way to convert rotary motion into linear motion.

Through a vast scope of screw formats and nut designs Abssac has been a preferred supplier of lead screws to key industries for well over three decades.

A lead screw, which is also often referred to as a power screw, is a length of metal bar stock that has had a thread form either cold rolled or machined into it.

Screw threads are available in a number of profiles including acme, stub acme, trapezoidal and buttress. All thread forms serve a purpose and so the specification of the correct screw thread type for an application is paramount.

Common to all lead screws, the thread form has what is termed a lead. This refers to the linear distance that can be travelled for each complete rotation of the screw. The mechanical advantage of a screw thread depends on its lead.

Additionally, all screws have a pitch, which is the linear (axial) distance between screw threads. For example, a thread pitch of 2mm means that the physical distance between one thread and the next is 2mm. These two terms (lead and pitch) are often used interchangeably, and for single-start screws (ie only a single thread helix), the lead and pitch are the same. However, it is important to remember that lead and pitch are not equal for screws with multiple starts - eg two-start, four-start, etc.

Example - vee thread form

Vee thread lead screws are excellent for finite movement within linear applications. The original vee form thread, has a 60-degree thread angle.

Example - trapezoidal thread form

The original square form trapezoidal thread, known as the Acme thread form, is the most commonly encountered and has a 29-degree thread angle. The metric version has a 30-degree thread angle.

To translate the rotation of the screw into a linear motion you require a nut. The nut's internal thread form should always match the external thread form of the screw for efficiency. How well these two items fit together will directly affect accuracy, efficiency and, of course, the backlash. The shape, function and material choice of the nuts is as vast as the screw selection. The challenge is matching the available form to the function, and this where Abssac excels. In nearly 40 years, Abssac lead screws have been chosen for applications all over the world and for various industry applications.

Captivating the nut on the screw (which is the most frequently used method) by means of a housing, or directly attaching the nut to the application, delivers the linear movement from the rotating screw. The physical load the screw can accept is proportional to the diameter of the screw and the length of travel. The nut size, or more importantly the size of the foot print (the touching surface between the threads) is also proportional to the available load capacity.

The nut material also plays a big part in the available load capacity and, of course, mechanical efficiency. Other parameters such as the speed of travel, duty cycle and the working environment are also considered when specifying. In short, the decision to use a lead screw and nut is a disciplined set of parameters that need to be reviewed to maximise the end result.

Abssac is a trusted partner in the development of lead screws and nuts with many projects refined alongside customers' requirements. Applications to date have seen miniature multi-start threads with a diameter of >1mm within medical scanners through to heavy-duty applications using diameters up to 140mm.

Offering a second-to-none machining capability, Abssac has been successful in many reverse-engineered applications, where the old part has been used as the template. This is particularly useful when engineering drawings of the required part no longer exist. The same machining capability also opens up the advantage of being able to supply lead screws with end journals pre-machined to accept bearing mounts.

For further products visit the Abssac website or contact one of the company's engineers.

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