Difference between double-headed bolts and fully threaded bolts
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Recent safety inspections require that all double-headed bolts be replaced with fully threaded bolts. I would appreciate some guidance – what is the main difference between them?
Fully threaded bolts are used less frequently; I’m not sure why you have such a requirement.
Fully threaded versions have higher strength and can be used in more demanding conditions; this is shown in the screenshot from 20592
Judging from the images found, a fully threaded one has threads from start to finish. The double-ended type has threads on both ends and a smooth shaft in the middle. There may be concerns that a bare shaft that is too long won’t be able to move once it reaches its intended position and also won’t be secure; whereas a fully threaded shaft can be screwed into any position
For flanges, I always use bolts with full threads.
Double-headed bolts are prone to stress concentration at the junction of the thread and the shank
I think it mainly comes down to the material used. The flange thickness of many large-scale equipment is extremely large; I’ve seen double-headed bolts that are a meter long, with double nuts on each end. The design drawings specify the use of double-headed bolts, and there’s no need to use fully threaded bolts – it’s a waste of manufacturing costs! Whether it is double-ended or fully threaded, as long as the nut is fully filled and tightened, the local stress concentration should be eliminated during the heat treatment process