Screw threads fall into two categories; The primary use is probably that of fasteners ie. nuts and bolts, included in this are connectors such as pipe fittings etc. The other use is as a positioning device such as a lead-screw. The thread forms utilized for these applications can be quite different; it is possible for the normal series of thread forms more typically,' used as fasteners, to be used for lead screws. However, it is not possible to use the thread forms designed for lead screws as fasteners.
Thread forms currently in use are:-
Threads which have been superseded but may still be encountered :-
Threads which are mainly used in the USA but may be found in the UK automobiles and aircraft.
Many countries including the UK had their own form of metric thread, but these have long been superseded by the ISO metric
series.
The ISO metric screw thread form is used in the UK these days, but supplies of nuts and bolts manufactured to the superseded
forms such as are still available to special order.
The British Standard Pipe thread has been adopted as the ISO standard pipe thread. Although the list of threads is in imperial
fractional sizes all other dimensions have been converted to the metric equivalent.
British Association threads are still used in instrument manufacture. This is because of the useful range available, not duplicated
by the ISO series.
Minor Diameter
Is the diameter of the roots of an external thread and the crests of an internal thread, sometimes referred to as the core diameter.
Pitch
Is the axial distance from a point (usually the crest) on a screw thread to a corresponding point on the adjacent thread.
Lead
Is the distance through which a screw thread moves axially in one revolution. On a single start thread the lead and pitch are
identical; on a two start thread the lead is twice the pitch; on a three start thread the lead is three times the pitch and so on:
For positioning devices especially heavily loaded lead screws, there are special types of thread form of which Acme and Square
are examples.
Another type of screw thread used in these applications is the recirculating ball screw; these are used where a reduction in friction
is an advantage, but because of their complexity, are more expensive. Recirculating ball screws can only be used as a unit, ie, the
nut cannot be removed,
Their operation is as follows:-
A number of ground balls are inserted between external and internal grooves of helical form; the balIs feed continually from one
end of the nut and circulate around between the internal and external grooves. As the halIs emerge from the grooves, they are
fed through a channel to re-enter the screw again, thus a continuous flow is set up which acts like a rolling bearing thereby reducing
friction.
Screw threads can be manufactured in most materials, but it should be noted that an external and internal thread designed to fit
together, should not both be manufactured in stainless steel. This material will pick-up and bind in this situation unless large
clearances or anti-sculling compound is used, It is quite likely that it will be impossible to dismantle any such components without
inflicting considerable damage.
There are various types of Bolt and Screw heads used in engineering, the following being examples:
Hexagon head
Hexagon shaped head - tightened with a spanner. Probably the most frequently used of all bolts and screws.
Socket (cap) head
Raised head with hexagon shaped hole in the centre Tightened with hexagon (allen) key. Useful where space does not permit
use of spanner also where head of bolt is recessed into component.
Countersunk head
Used where the bolt or screw head needs to be flush with the surface; a countersunk hole needs to be provided. Two types are
commonly used; one with a slot for a screwdriver the other with a hexagonal hole for tightening with an Allen key.
The above types are those most commonly used, but there are others which are generally for use in light engineering; such as round head, mushroom head and cheesehead.
Set screws
These are used to prevent two parts in relation to moving one another. This is achieved by a pointed end on the screw "digging in"
and preventing relative movement.
Shouldered bolts
Used when the depth to which a bolt may be screwed in needs to be restricted. Mainly used when the component held by the bolt
must not be tightly clamped or the clamping force needs to be restricted. The shoulder will control the position of the bolt head.
Stiffnuts
Various types of stiffnut are available, some are manufactured with a number of threads deformed which grip the bolt thread;
another type locks by means of an unthreaded nylon or fibre insert into which the bolt or screw will cut it's own thread, thereby
providing a grip.
Slotted or castle nuts
These are provided with slots and when the nut is in it's tightened down position, the bolt or screw is drilled in line with one of
the slots in the nut and a split pin inserted.
There are other forms of nuts such as those which are square in outline, but it is unusual for any of these to be used in engineering applications.
Mechanical Engineering Design Topics Page
Pages created by David Price - 9/08/95
This Information is an educational resource and is not to be reproduced (other than for private study) without
written consent of the University of Southampton. copyright © Dave Whatley 1995
GlobalSpec.com is presenting this material with the consent of Dave Whatley.
Disclaimer Whilst every care has been taken to ensure the accuracy and completeness of this information, neither the author nor the University of Southampton will accept responsibility for loss, damage etc. however arising occasioned by the use of this information.