Fastner with Captivated Washer

You can use screw and washer with different hardness; -it should be different hardness in screw and washer, the latter to be the hardest one. Our company has used this kind of screw/washer produced by BULTEN STAINLESS INDUSTRY AB, Sweden.

Do you need a pre-assembled SEMS screw like these http://www.sems-uk.co.uk/117_What_are_Sems_Screws.asp or do you need to retain the washer for assembly with a push on retainer like this http://products.jetpress.com/products/3012/010/ ?

Yes, I am looking for Sems screws.

Do you have any idea how they are manufactured. Basically, I would prefer buying from a local manufacturer than importing them. And one of the manufacturer is suggesting that the screw & washer will be first assembled & then Heat Treated, now since washer & screw are having different hardness, the process seems doubtful.

Pls guide.

SEMS is a generic name for this type of screw, they are made by lots of different companies. It would be be best to contact several local suppliers to see if they can supply what you need. They could get different properties in one process by using two different alloys but, if you have doubts, ask them to supply samples for you to test.

Hi,

I used to work for a company that made their own screw/washer assemblies of this type.

They worked by having the hole through the washer tapped at the same size as the screw and having an undercut at the head end of the screw to the root diameter of the thread. The undercut needs to be slightly wider than the washer thickness to allow the washer free movement when it screws off the end of the thread.

I hope this is of some help.

Drew

Typically, in high volume, as screws are usually made by cold heading, the washer, which itself may be very hard, would be fed onto the screw blank and positioned in the die between where the head is formed and where the threads are formed. This then captures the washer between two larger diameters.

Another way, and one more suitable for lower volumes and screws which may be cold-formed or machined, would be assemble the screw and washer, then place the assembly into a press, swager, or other cold-working machine that "upsets" the screw diameter at one point just below the washer when it is flush against the head, deforming the diameter so the washer can no longer pass freely over it and fall off.

Both these methods require the screw to be of sufficient hardness and temper to allow cold-forming without cracking, as would happen with an extremely hard or brittle material. Steel having a hardness of 35 HRC, depending on the alloy, is possible to cold work, if given sufficient pressure.