Strap clamps

In an ideal rigid strap, a 12 kN downward stud load between contacts 90 mm apart gives 8 kN at the workpiece and 4 kN at the heel when the stud is 30 mm from the workpiece. At 45 mm, the reactions are 6 kN each. The static model excludes pad compliance, friction and cutting loads.

Use Choosing workholding to compare location, support and restraint across fixture types.

The strap as a loaded beam

The strap spans a workpiece contact and heel support. The stud or screw loads it between those contacts.

Carr Lane treats the strap as a beam in bending. Its guide covers slotted heels, tapped heels, double-end straps and step clamps. Carr Lane strap-clamp guide, mechanical principle and types

Components and their functions

Carr Lane's fixture-design guidance separates locating the workpiece from restraining it. Clamping into a supported, stronger region limits bending; locating elements can react machining loads instead of relying exclusively on friction. This does not imply that a clamp's catalog force alone establishes a fixture's cutting-load capacity. Carr Lane locating and clamping principles

Heel arrangements and product examples

A slotted heel permits a strap to slide clear for unloading. A tapped heel can carry an adjustable rest or a clamping screw. Step-clamp rear serrations mate with a corresponding support block; screw rests and solid blocks provide other rear-support arrangements. Springs around a stud can raise the released strap for loading, but are not the principal clamping-force source. Carr Lane strap-clamp guide, heel supports and accessories

Carr Lane's Plain Clamps are double-end straps with tapered noses. The manufacturer describes either simultaneous clamping of two workpieces or single-workpiece use with a rear Step Block. This family accommodates 3/8-16 through 3/4-10 studs, or M10 through M20 metric variants, with three strap-length choices per stud size. Its stated material is carburized-hardened 11L17 steel with black oxide. Those details identify this product family, not the material or capacity of every plain strap. Carr Lane Plain Clamps

The manufacturer's separate Step Blocks can work as an interlocking pair, or singly with a Step Clamp. It specifies matching serrations within its own block/clamp range and lists standard, heavy and extra-heavy widths of 1, 1½ and 2 inches. That compatibility statement does not establish interchangeability with another manufacturer's tooth pitch. Carr Lane Step Blocks

Worked example: where the stud load goes

Original idealized static model: a rigid strap has its workpiece contact at x = 0, rear heel at x = L, and downward stud load P at x = a, where 0 < a < L. Contacts provide upward reactions to the strap. Neglecting weight, friction and deflection:

Workpiece force magnitude F = P × (L − a) / L
Heel reaction magnitude R   = P × a / L
F + R                       = P

For an assumed P = 12 kN, L = 90 mm and a = 30 mm, the workpiece receives 8 kN and the heel reacts 4 kN. Moving the stud to a = 45 mm gives 6 kN at each end at the same preload. Moving it toward the workpiece increases that end's share; it does not increase stud tension or establish the strap's strength.

The calculation requires stable contacts and the stated geometry. A double-end arrangement with two workpieces uses their contacts as the two supports; equal force follows in this model only when the actuator is centered between them. It does not follow merely from having two noses. Strap bending, contact migration and different support stiffness can change the real distribution.

Contact, tightening and access

Original contact illustration: 8 kN distributed uniformly over a 20 × 10 mm pad gives an average pressure of 40 MPa. A 20 × 5 mm contact gives 80 MPa at the same load. These are nominal averages; edge contact, surface shape and elastic deformation can produce higher local stresses. Neither number predicts allowable pressure or permanent marking for an unspecified material.

As engineering synthesis, nut torque is an input to a friction-dependent threaded joint, not a direct workpiece-force measurement. Thread condition, lubrication, washer seating and the lever dimensions intervene. A torque value for another stud or clamp cannot be converted to a common force without those assumptions. Likewise, a heel that rocks or a nose pressing across an unsupported thin wall changes the intended load path.

The top strap and exposed fastener occupy machining space. A longer reach changes bending and clearance; a thinner nose changes contact geometry and section. These relationships distinguish the arrangement from an edge or toe clamp intended to leave more of the upper face accessible.

Coverage limits

The cited web pages establish no technical issue dates. Named materials, dimensions and interface claims apply only to the specified manufacturer families. This reference establishes no universal tightening torque, stud engagement, permissible overhang, strap inclination or cutting-force rating. The complete fixture includes the strap, fasteners, base, supports and workpiece.

See Workholding and fixture references for locating and restraint concepts.