A C-clamp closes a load path through its fixed anvil, workpiece, swivel pad, screw and frame. Turning the spindle advances the pad through the threaded boss. This calculator estimates axial force from torque, screw lead and an assumed efficiency; the animation shows approach, contact and release.
C-clamp Interactive Calculator
Change torque, screw lead and effective efficiency to estimate contact force. The screw animation advances one lead per revolution, pauses at contact, then reverses. The result is a force estimate, not the rated capacity of a particular clamp.
Equation Used
- Per-revolution work balance: input work is 2πT and useful axial work is F times screw lead.
- Effective efficiency represents losses included by the user; no additional loss deduction is applied.
- Rigid illustrative frame and workpieces; the animation does not calculate elastic strain or compression.
- Results describe an estimated load after contact. A gap is unloaded, and product load ratings are not inferred.
The opposing jaws carry equal reactions in this static model, while the C-frame carries bending as well as force. Entered efficiency is an assumed effective value, not a measured product characteristic.
The interactive animation above shows a screw-driven C-clamp approaching the workpiece, making contact and releasing.
How the clamp closes its load path
The screw runs through a threaded boss at one end of the C-frame. Turning it advances a pressure pad toward the fixed anvil at the other end. A swivel connection lets the pad remain aligned with the workpiece while the spindle turns.
After contact, the opposing jaw forces are equal in this static model. The curved frame carries a combination of force and bending; it is not a member in pure axial tension. The frame, threaded boss, spindle, pressure pad and workpiece form the connected load path.
What the force estimate is useful for
The calculator illustrates why more torque or a smaller screw lead can increase axial force. It can compare assumptions for a screw-driven clamp. Real clamping also depends on contact surfaces, stiffness, condition and the particular tool’s rated limits.
BESSEY’s catalogue shows several C-clamp constructions and different operating heads. The illustrated sliding-bar handle is one arrangement, not a claim that every C-clamp uses that handle or thread.
Force from work per revolution
One spindle revolution takes input work of 2πT, where T is torque. The screw advances by one lead L, so useful axial work is FL. With selected effective efficiency η, FL = η2πT. Thus F = 2πηT/L.
For the displayed units, multiply torque in ft·lb by 12 to obtain in·lb before dividing by lead in inches per revolution. The no-loss comparison uses η = 1. Jaw reaction equals the estimated axial force. Loss fraction is 100(1 − η).
Efficiency is an assumption, not a substitute for a measured tool characteristic. This calculation does not determine self-locking or solve the frame’s bending stress.
Try the default screw-force comparison
At 10 ft·lb and 0.125 in per revolution, the ideal no-loss force is about 6,032 lbf. With the selected effective efficiency of approximately 16.58%, the estimate is about 1,000 lbf.
Double torque while holding the other inputs fixed and estimated force doubles. Double screw lead and the estimate halves. The screw also advances twice as far per revolution in the animation. These comparisons do not establish a safe torque or load for a physical clamp.
Force, travel and structural limits
A smaller lead gives less axial travel per turn and more ideal mechanical advantage. Actual efficiency can change with geometry and friction; keeping it fixed is a comparison assumption.
The calculator does not know the frame section, throat depth, material, spindle size or contact area. It therefore cannot predict deflection, contact pressure, failure load or tool capacity. Use the actual manufacturer’s ratings for the selected clamp.
Questions about the C-clamp model
Why does the screw move without squeezing the workpiece in some frames?
Those frames show unloaded approach or release. Force results apply after contact at the selected torque.
Does the frame carry only tension?
No. The offset jaw forces also bend the C-frame. The result card reports jaw reaction, not frame stress.
Is the efficiency a measured value for a named clamp?
No. It is an adjustable effective assumption used in the work-balance calculation.
Does a larger calculated force mean my clamp can carry it?
No. The model does not check the tool’s structural capacity or rated limits.
Construction reference
BESSEY C-clamp catalogue: photographs of C-frames, threaded spindles, pressure ends and operating-head variants. The drawing is illustrative and the force model is a separate elementary work-balance calculation, not a BESSEY product rating.
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