Page 22 - IT_June_2020_Classical

MACHINE BUILDING

LEADING THE WAY

LEAD SCREWS AND NUTS

THE COMBINATION OF A LEAD SCREW AND NUT IS AN EXCELLENT
AND ECONOMICAL WAY TO CONVERT ROTARY MOTION INTO LINEAR
MOTION, AS THE EXPERTS AT ABSSAC EXPLAIN

lead screw, which can also be known as a power
screw, is a length of metal bar stock that has had
a thread form, either cold rolled or machined
Ainto it. The screw thread form is then available
in a number of profiles including terms such as acme, stub
acme, trapezoidal and buttress. All thread forms serve a
purpose and so the specification of the correct screw for
an application is paramount.
Common to all lead screws, the thread form has what
is termed a lead. The lead of a screw refers to the physical
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
physical 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 terms 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, for example
2-start, 4-start, etc.

Accuracy, efficiency and backlash
To translate the rotation of the screw into a linear motion
you require a nut. The nuts 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 touching surface between the threads) is also proportional
matching the available form to the function and this where to the available load capacity.
Abssac excels. In nearly 40 years, Abssac lead screws The nut material also plays a big part in the available
have been chosen for applications all over the world and load capacity and of course mechanical efficiency. Other
into various industry applications. parameters such as the speed of travel, duty cycle and the
Captivating the nut on the screw (which is the most working environment are also considered when specifying.
frequently used method) by means of a housing or directly In short, the decision to use a lead screw and nut is a
attaching the nut to the application, delivers the linear disciplined set of parameters that need to be reviewed to
movement from the rotating screw. The physical load maximise the end result.
the screw can accept is proportional to the diameter of Abssac is a trusted partner in the development of lead
the screw and the length of travel. screws and nuts with many projects refined alongside
The nut size, or more importantly customers’ requirements. Applications to date have seen
the size of the miniature multi-start threads with a diameter of around
foot print (the 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 are no longer in existence.
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.
MORE INFORMATION: www.abssac.co.uk

22 INDUSTRIAL TECHNOLOGY • May/June 2020