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Guide to Grub Screws

Grub screws

What is a grub screw?

A grub screw is a type of headless screw used in precision applications where a discreet locking solution is required for an assembly. Grub screws employ a machine type thread to achieve this, for use with a tapped hole.

View all available Grubs Screw sizes and materials

What are Grub Screws used for?

Grub screw used on a pillar drill pulley In an industrial environment grub screws are commonly used for locking components into position on shafts. The absence of a head ensures that the screw can sit descreetly below the surface of the assembled component.

Grub screws are commonly seen in door locking mechanisms and handles, and can be found domestically in bathroom fittings, curtain rails, light fittings and taps. Grub screw used on a door handle mechanism

Are there other names for Grub Screws?

Grub Screws have a number of alternative names, including:

  • Set Screws or setscrews
  • Socket Set Screws
  • Blind Screws

Please note that the phrase "set screw" can also refer to a fully threaded hex bolt in some markets around the world.

How to secure a Grub Screw?

Grub screws are secured using a hex or allen key, but some types will require a slotted screwdriver. Other drives include Torx or 6 lobe , and square socket (known as a Robertson drive). Grub screw example application

How do you measure a Grub Screw?

Grub screw length measured with a ruler Grub Screw length is measured from end to end, including the point. The length of a grub screw can be measured perfectly satisfactorily with a standard ruler or tape measure. The example shown in the image is 25mm in length.

The diameter of a Grub Screw is best measured with vernier calipers or a micrometer and will be slightly less than the nominal diameter. The M10 standard coarse pitch example shown in the image will measure between 9.732mm and 9.968mm. Grub screw diameter measured with vernier calipers

What types of Grub Screw are there?

Grub screws are available in a number of different types. The 4 most common profiles or types are:

  • cone point
  • flat point
  • dog point
  • cup point
Cone point grub screw Cone point grub screws feature a pointed end that is an ideal shape for use with softer materials or for being inserted into an existing depression. Cone point grub screws with metric threads conform to DIN 914 and ISO 4027.
View Cone Point Grub Screw sizes and materials

Flat point grub screw Flat point grub screws achieve the largest flush contact with the adjoining part due to their flat end. Flat point grub screws with metric threads conform to DIN 913 and ISO 4026.
View Flat Point Grub Screw sizes and materials

Dog point grub screw Dog point grub screws include a smaller diameter shaft or spigot, protruding from one end. This is intended to be used as a spindle or pivot which the adjoining component can swivel around. Dog point grub screws with metric threads conform to DIN 915 and ISO 4028.
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Cup point grub screw Cup point grub screws include a concave depression at one end, offering a similar function to the cone point, but can be driven in further, providing more torque and gripping force to the adjoining surface. The cup point is the most widely used profile. The curved edges of the cone spread the contact area, lessening the stress on the adjoining surface. Cup point grub screws with metric threads conform to DIN 916 and ISO 4029.
View Cup Point Grub Screw sizes and materials

Cup knurl point grub screw Knurled cup point grub screws are the knurled cousin of the standard cup point grub screw. The knurled feature helps reduce loosening from vibration. The knurls consist of angled ridges on the edge of the cup surface that, when tightened, have a ratchet-locking action as they cut into the mating component. Knurled cup point grub screws cannot be effectively reused as the cutting edges of the knurls are deflected or deformed when tightened on fitting.

Thread pitch types of grub screws

In addition to the metric threaded items, grub screws are available in Unified UNC (coarse pitch) and UNF (fine pitch). The ASME standard B18.3 defines this specification. Grub screws are a well established fastener type, and so are likely available in other thread standards. Metric, UNC and UNF grub screw thread forms
View all Grub Screw sizes and materials
View UNC Cup Point Grub Screws sizes and materials
View UNF Cup Point Grub Screws sizes and materials

How Strong are Grub Screws?

Grub screws are not rated for tensile strength like other threaded fasteners, as they are designed for compression rather than tensile loading.

Instead they are given a hardness rating. The Rockwell hardness measure is used to express the strength of a grub screw. 45H is the standard hardness for steel grub screws manufactured to the relevant ISO or DIN standard mentioned above, whereas stainless steel equivalents are less hard in comparison and rated 21H according to the same standard.

The strength specifications of grub screws manufactured to ASME B18.3 are defined in ASTM F912 for steel alloy and ASTM F880 for stainless steel.

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