Hobson tightening torque chart
SOKO UNC Socket Screw Torque Chart: ASTM A574 Recommended Tightening Torque
Hobson's one-page chart of recommended tightening torques for the SOKO range of UNC (imperial coarse) socket screws, socket head cap screws to ASTM A574 (1960 series). Torques are in pound-feet, plain and zinc plated, for socket head, low head, countersunk, button head, set (grub) and shoulder screws.
What this covers
- Socket head
- Low head
- Countersunk head
- Button head
- Socket set screw (grub)
- Shoulder screw
Applies to Socket Button Screw, Socket Head Cap Screw, Socket Countersunk Screw, Grub Screws (Socket Set Screws) and Socket Shoulder Screw.
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 1-1/2 inch.
- Socket head cap screw, plain (zinc plated): 1/4 inch 17.0 lb-ft (11.9), 3/8 inch 62.0 (43.4), 1/2 inch 150.0 (105.0), 5/8 inch 283.0 (198.1), 3/4 inch 500.0 (350.0), 1 inch 1,040.0 (728.0).
- Zinc plated screws get about 30% less torque than plain ones, because zinc plating lowers the friction.
- 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 UNC socket screws.
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