Hobson tightening torque chart
SOKO UNF Socket Screw Torque Chart: ASTM A574 Recommended Tightening Torque
Hobson's one-page chart of recommended tightening torques for the SOKO range of UNF (imperial fine) socket screws, socket head cap screws to ASTM A574 (1960 series). Torques are in pound-feet, plain and zinc plated, for socket head, countersunk head, button head and set (grub) screws.
What this covers
- Socket head
- Countersunk head
- Button head
- Socket set screw (grub)
Applies to Socket Button Screw, Socket Head Cap Screw, Socket Countersunk Screw and Grub Screws (Socket Set Screws).
Key things to remember
The essentials from this document in plain words. Check the full document before you rely on a figure.
- Socket head cap screws are listed from #0 to 1 inch.
- Socket head cap screw, plain (zinc plated): 1/4 inch 19.0 lb-ft (13.3), 3/8 inch 70.0 (49.0), 1/2 inch 172.0 (120.4), 5/8 inch 317.0 (221.9), 3/4 inch 562.0 (393.4).
- Countersunk and button head screws take about half the socket head torque, for example 83.3 lb-ft plain at 1/2 inch.
- Conditions: a guide only, for the first tightening of a single assembly on its own, using a k factor of 0.2 for plain threads (burr-free, lightly oiled) and 0.14 for zinc plated.
- Torque is only a rough way to set bolt tension: the chart does not account for the many real-world variables that change friction.
Who it helps: Fitters working on imperial machinery with UNF socket screws.
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