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What is a set screw?

clock 6 minutes | 07 Feb 2022

Pile of black set screws

Set screws are not commonly used, as they have a specialist function: Secure one component inside another component. Set screws are ideal for this, as they lack protruding parts that would interfere with the two components’ movement, as is necessary for gears on shafts.

Set screws are easy to identify. As shown here, they’re threaded from top to bottom. They’re not headless as sometimes described – they do contain the drive. The example here shows a set screw slotted, but most are hex set screws, meaning they have a hex drive. They’re also called socket set screws. All socket screws have hex drives and are secured with allen, or hex, keys. Learn more about socket screws. We explain all drive types in our guide What are screw heads, drives and threads?

Set screw slotted

What are set screws used for?

Set screws are a type of blind screw, designed to hold collars, pulleys, or gears on shafts. If you used a hex bolt, you’d most likely find that the assembly failed to work efficiently and smoothly due to resistance from the bolt’s head. A set screw, used without a nut, is strong enough to keep the assembly secure as intended, but with the additional benefit of staying out of the way.

Set-screw uses also include ironmongery and joinery – particularly door-knob set screws and door set screws.

How do set screws work?

Traditional screws hold components together by their threads. Set screws are threaded from top to bottom, but they work differently. They exert compressional force through their tip. This is what stops the relative movement between the two components, allowing them to function as designed.

Set screws vs. bolts

Typical screws are driven into a surface to create a new hole. Bolts fit into a tapped hole. This is where set screws are similar – they’re also driven into a tapped hole. Both also maintain the same diameter along their length. For these reasons, some people consider set screws to be a type of bolt. Certainly set screws can do the work of bolts in some applications, such as the example we’ve already given. A key difference between bolts and set screws: bolts have protruding heads while set screws do not, which enables set screws to sit flush with a surface.

Grub screws vs. set screws

What is a grub screw? It’s really a different name for a set screw, but there are different views on this. The difference to some people lies in how they’re used. If the entire screw sits inside a hole – as most set screws typically do – then it’s sometimes referred to as a grub screw. Another opinion: if the screw has a hex drive, then it’s a set screw. If another type of drive, such as slotted, then it’s a grub screw. To others, it’s always a set screw and still to other people, it’s always a grub screw. There is no hard and fast consensus.

Set screw in application

Set screw types

Below are the different points available. Point penetration contributes as much as 15% to the total holding power. The cone point requires the greatest installation torque due to its deeper penetration, while the oval point gives you the smallest increase in holding power.

Dog point set screw

Dog point set screw

  • Tubular, flat tip mates with pre-drilled holes, allowing shaft to rotate without displacing part
  • Extended tip is typically used to locate into a machined groove on a shaft
  • Can replace dowel pins

Half-dog set screw

Half-dog set screw

  • Also known as extended tip set screws
  • Shorter protrusion than dog point
  • Designed for permanent setting – protrusion fits into a matching hole
  • Flat tip spans the entire screw, helping to locate a machined groove on a shaft
Cup point set screw

Cup point set screw

  • Cup-shaped tip cuts into surface, preventing component from loosening
  • Design provides good vibration resistance
  • Leaves a ring dentation mark on surface
  • Concave, hallowed end
Cone point set screw

Cone point set screw

  • Cone set screws generate highest torsional holding power
  • Penetrates into flat surfaces
  • Acts as a pivot point
  • Ideal for exerting more force when joining softer materials together
Nylon point set screw

Nylon point set screw

  • Soft nylon tip grips curved or textured surfaces
  • Nylon set screw can match the shape of mating surface
  • Suited for applications that need a tight fixing, without marking the mating surface
  • Can be used for round shafts and uneven or angled surfaces
Oval point set screw

Oval point set screw

  • When installed the point minimises surface damage at the contact site
  • Small contact area enables slight adjustments without loosening the screw
  • Oval set screws are ideal for applications that need adjustments on regular basis
Knurled cup point set screw

Knurled cup point set screw

  • Serrated ridge on knurl cup set screws dig into surface, reduce loosening due to vibration
  • Can not be reused due to the deflection of the knurl’s cutting edges when tightened
  • Also ideal for joinery applications
Flat point set screw

Flat point set screw

  • Flat set screws enable evenly distributed compression force where the tip makes contact with a surface least penetration to the target surface so give less holding power
  • Can be used against a thin wall or soft material
  • Used when frequent adjustments are required

How to determine set screw size

When choosing set screw sizes, consider the holding power provided by the set screw clamping force. Point penetration provides some additional resistance to rotation. As a general rule of thumb, holding power is defined as the tangential force in pounds, as design considerations will result in different sizes of shafts being used with a certain set-screw size.

You’ll choose your set screw size based on the screw’s diameter, which should be equal to roughly one-half shaft diameter. That said, always check the manufacturer’s data on the screw.

Choosing your set screw

Below is a comparison of the different types of set screws:

Dog

Half dog

Cup

Cone

Nylon

Oval

Knurled cup

Flat

Permanent hold







High compression force


Reusable without damaging threads and surface



Good fit with holes



Good fit with grooves



Can be used as adjusting screw



Can be used as hanger points


For use on both hard and soft materials





Can be used at any angle




Which material?

The most common materials for metal set screws are brass, alloy steel and stainless steel. Nylon set screws are the popular choice for plastics. The table below tells you something about their characteristics.

Priority

Plastics

Stainless steel

Alloy steel

Brass

Strength




Lightweight



Corrosion resistant





Download free CADs and try before you buy

Free CADs are available for most solutions, which you can download. You can also request free samples to ensure the threaded socket screws you’ve chosen are exactly what you need. You might find The ultimate guide to fasteners helpful. If you’re not quite sure which product will work best, our experts are always happy to advise you.

Whatever your requirements, you can depend on fast despatch. Request your free samples or download free CADs now.

Questions?

Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.

Set screw application
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