Guide to Strap Clamps

A clamp strap is the simplest and most common clamp. The basic clamp-strap assembly consists of three major elements: a clamp strap, a fastening device, and a heel support, Figure 8-1. Force is applied to the fastening device. The force is then transferred through the strap to the workpiece. The heel support acts as a pivot and support for the back end of the strap.

Clamp-Strap Operation

All clamp straps work on the mechanical principle of the level. As shown in Figure 8-2, the three basic styles of strap clamps can be described in terms of lever arrangements, called first-, second-, and third-class levers. The classes are not meant to describe importance or preference levels; rather they show distinctions in the mechanical actions of each lever style.

Major elements of a clamp strap assembly
Figure 8-1. The major elements of a clamp strap assembly are the clamp strap, fastening element or assembly, and heel support.

As shown, the major difference between the three strap-clamp variations lies in the arrangement of the three main elements in each lever. These elements are the force-applying device, the workpiece, and the fulcrum.

As shown in Figure 8-2(a), a first-class lever has the workpiece at one end, the fulcrum in the center, and the force at the opposite end of the clamp strap. A second-class lever, shown in Figure 8-2(b), has the fulcrum at one end, the workpiece in the center, and the force at the other end of the clamp strap. Finally, as shown in Figure 8-2(c), a third-class lever places the part at one end of the clamp strap, the force in the center, and the fulcrum at the opposite end. Each of these arrangements is well suited for certain workholding situations. With strap claps, the third-class-lever arrangement is the most common.

Clamping force on the workpiece depends on the relative position of the workpiece, fastening element, and fulcrum. All strap clamps are basically beams that are loaded in bending, as shown in Figure 8-3. The loads on a clamp strap are the applied force F, clamping force P, and reaction force R. The applied force is the force applied by the fastening device. For most calculations, this force is known. The clamping force is the actual force applied to the workpiece during the clamping operation, a fractional portion of the applied force. The reaction force is the force generated on the fulcrum. Changing the positions of the various elements affects the amount of clamping force.

Strap clamps
Figure 8-2. Strap clamps can be categorized as first-, second-, or third-class levers.
Three classes of levers
Figure 8-3. Force diagrams show the difference between the three classes of levers.

The value L1 is the distance between the fulcrum and the applied force. L2 is the distance between the fulcrum and the part-contact point. These values determine the force ration (mechanical advantage) of a clamping arrangement. The actual clamping force applied to the workpiece with each setup is a proportional amount of the applied force. The ratio of actual clamping force to applied force is equal to the ration of the two distances:

P/F = L1/L2

With a third-class lever arrangement, as shown in Figure 8-4, the force on the workpiece depends on the position of the stud with respect to the workpiece and the heel support. If the stud is positioned exactly in the center of the clamp strap, the force generated by the fastener is distributed equally between the workpiece and the heel support. If two workpieces are clamped with a single strap-clamp arrangement, as shown in Figure 8-5, positioning the stud in the center of the clamp strap applies equal holding force to both parts. In Figure 8-6, the fastener is positioned so that only one-third of the clamp-strap length is between the fastener and workpiece, while two-thirds is between the fastener and the heel support. The clamping force on the workpiece with this setup is twice as great as that on the heel support.

P/F = 1/2
Three classes of levers
Figure 8-4. Positioning the stud in the center of a third-class lever distributes the force equally between the workpiece and heel support.
positioning the stud in the center of the clamp strap
Figure 8-5. When clamping two workpieces, positioning the stud in the center of the clamp strap applies equal holding force to both workpieces.
positioning the stud in the center of the clamp strap
Figure 8-6. Positioning the stud closer to the workpiece generates proportionally more force on the workpiece and less on the heel support.

Types of Clamp Straps

Clamp straps come in a variety of forms. Selecting the correct clamp strap for a task is important to a workholder’s operation and efficiency. Clamp straps vary both in material and basic form or style. The most common material for clamp straps is carburized-hardened carbon steel, but forged steel, stainless steel, and aluminum clamp straps are also available.

The specific material used for a particular workholder is normally determined by the application itself. Steel clamp straps are normally used for general purpose applications. Stainless steel clamp straps are used when more corrosion resistance is needed. Forged steel clamp straps are normally employed for applications where extra toughness is required. Aluminum clamp straps are often used where weight is important or where softer clamps are needed to prevent marring workpiece surfaces.

Clamp straps can also be classified based on the form of the strap. Variations in the end shapes, contact areas, and heel supports are all used to classify clamp straps. Like the material, these factors can greatly influence the selection and application of these clamps.

The most common clamp-strap shapes are shown in Figure 8-7. They include the plain strap (a), tapered-nose (b), wide-nose strap (c), U-shaped strap (d), gooseneck strap (e), and the double-end strap (f). The variety of end shapes provides clamp straps tailored to exact clamping requirements and conditions.

Clamp straps
Figure 8-7. Clamp straps are made in a variety of shapes. Most common are the plain, tapered-nose, widenose, U-shaped, gooseneck, and double-end straps.

The workpiece contact area of the clamp strap is also an important consideration in clamp selection. The contact areas, Figure 5-8, can be either flat or radiused. The radius nose (a) is usually best because it keeps the contact area to a minimum. A small contact area best avoids warping the workpiece. The flat contact (b) offers broader contact with the workpiece, for less contact pressure. A variation of the flat contact is the padded clamp strap (c). Padded straps come with a steel pad, but brass or plastic pads are also available. As shown in Figure 8-7, the force applied is measured in pounds per square inch. When the contact area is reduced, the force is increased. When the contact area is less than one square inch, the applied force multiplies rapidly.

Clamp straps
Figure 8-8. Clamp straps are available with different workpiece contact areas. The most often used are radiused, flat, and padded nose.

In addition to a variety of forms and contact areas, clamp straps are also made with several different types of heel supports. The heel support area of a clamp strap is at the end opposite the contact area. Figure 8-9 shows several heel supports. The most basic heel support shape is the plain heel (a). The plain heel, as shown here, frequently uses step blocks as a support. A variation of this heel support also takes a step-block approach, the stepped-heel clamp strap (b). This clamp incorporates the same pattern of steps into the end of the strap. Only one step block, rather than two, is required to support the end of the clamp. The tapped-heel clamp strap (c) uses a threaded heel support for adjustment.

Clamp straps
Figure 8-9. Various heel supports are used for clamp straps. The most common clamp heels are the plain, stepped, tapped and slotted heels.

The last type of heel support is the slotted-heel type (d). The slotted-heel clamp strap has a shallow slot cut into the underside. This slot aligns with a clamp rest screw, which adjusts to suit the height of the workpiece. This sliding feature permits the clamp to be easily moved on and off the workpiece, while returning to the same clamping position relative to the workpiece.

Although each area of the clamp strap has been discussed separately here, in practice these elements are combined into a variety of standard clamps. The more common types are as follows:

Slotted-Heel Clamp Straps. The slotted-heel clamp strap, Figure 8-10, has a slotted heel to locate the clamp strap on the clamp rest. This slotted heel, in combination with the slotted stud hole, allows the clamp strap to move completely clear of the workpiece for part removal and reloading. These clamps are made in many lengths and in stud sizes from #10-32 through ¾-10, in either carbon or stainless steel. The contact area of these clamp straps is available in radius-nose, plain-nose, and padded-nose styles. Slotted-heel clamp straps are also available in a gooseneck version, Figure 8-11.

Slotted-heel clamp strap
Figure 8-10. The slotted-heel clamp strap uses a slot to guide the strap on the clamp rest. This design permits the clamp strap to be moved completely clear of the workpiece, for easier loading/unloading of workpieces.
Clamp straps
Figure 8-11. The gooseneck version of the slotted-heel clamp strap has a recessed clamping nut to provide a low overall profile.

Variations of this clamp strap are used on high-rise clamps, show in Figure 8-12. These assemblies can be mounted on riser blocks for tall workpieces. They come in the three basic styles shown here, with a standard slotted-heel clamp strap, a tapped-nose clamp strap, or a gooseneck clamp strap. The tapped-nose version has a tapped hole at the clamping end, for mounting contact elements or adjustable spindles. The gooseneck clamp strap is used to lower the height of the fastening element.

Clamp straps
Figure 8-11. The gooseneck version of the slotted-heel clamp strap has a recessed clamping nut to provide a low overall profile.

Variations of this clamp strap are used on high-rise clamps, show in Figure 8-12. These assemblies can be mounted on riser blocks for tall workpieces. They come in the three basic styles shown here, with a standard slotted-heel clamp strap, a tapped-nose clamp strap, or a gooseneck clamp strap. The tapped-nose version has a tapped hole at the clamping end, for mounting contact elements or adjustable spindles. The gooseneck clamp strap is used to lower the height of the fastening element.

Clamp straps
Figure 8-12. High-rise clamp assemblies are ideal for clamping taller workpieces. These all-in-one assemblies can be stacked on risers for additional height.

Tapped-Heel Clamp Straps. The tapped-heel clamp strap has a tapped hole in the heel end of the strap. The component installed in this hole can be either a screw clamp that applies clamping force, Figure 8-13, or an adjustable heel rest, Figure 8-14. When used to mount an adjustable heel rest, the complete clamp is a third-class-lever arrangement, but when used to mount a screw clamp, it then employs a first-class-lever action.

Like slotted-heel clamp straps, tapped-heel straps are made in many sizes, of either carbon or stainless steel. The contact area of these clamp straps is also available in radius-nose or padded-nose styles. Two other end variations include the tapered-nose and wide-nose styles, Figure 8-15. The tapered nose works well when space is limited. The wide nose is used where a larger contact area is needed either to hold the workpiece or to spread out the clamping force.

Clamp straps
Figure 8-13. The tapped-heel clamp strap has a tapped hole in the heel to mount a clamping screw. A slotted guide block allows the strap to slide straight back and forth.
Clamp straps
Figure 8-14. Tapped-heel clamp straps can also be used with an adjustable heel rest, such as a leveling foot.
Clamp straps
Figure 8-15. Tapped-heel clamp straps are also available with a tapered nose or wide nose.
Clamp straps
Figure 8-16. Tapped-heel clamp straps are also made in either forged steel or aluminum alloy, in straight and gooseneck styles.
Clamp straps
Figure 8-17. Forged tapped-heel clamp straps are available with a locator nose for precise clamp position.

The forged tapped-heel clamp strap, Figure 8-16, is a variation of the tapped-heel clamp strap made in either forged steel with a 70,000-psi yield strength, or in aluminum alloy with a 25,000-psi yield strength. Both varieties are made in either straight or gooseneck styles, with either a plain nose or a locator nose. The locator-nose clamps, Figure 8-17, have an accurately machined locating notch to ensure precise, repeatable clamp positioning. This is ideally suited where reduced clamp contact is needed for machining clearance.

Double-End Clamp Straps. The double-end clamp strap holds workpieces on both ends. As shown in Figure 8-18, this clamp strap has a radius nose on both ends. This design provides equal clamping force on both ends, even if there are height variations in the workpieces. Double-end clamp straps are also available in a gooseneck version, Figure 8-19.

Double-end clamp strap
Figure 8-18. The double-end clamp strap is designed to clamp on both ends and applies equal force to both workpieces.
Double-end clamp strap
Figure 8-19. The gooseneck version of the double-end clamp strap has a recessed clamping nut to provide a low overall profile.

Step Clamps. Step clamps are one of the more common clamp strap designs. These clamp straps have a series of serrations, or steps, machined in their heel, Figure 8-20(a). These steps are designed to engage a similar set of steps in a matching heel block, Figure 8-20(b). This design allows the clamp to be positioned for different workpiece heights.

Double-end clamp strap
Figure 8-20. Step-type clamp straps have serrations machined in their heel to engage a matching heel block.

Forged U Clamps. The forged U clamp, Figure 8-21(a), is a general purpose clamp strap. As shown in Figure 8-21(b), both the U end and pin end of the forged steel clamp strap can clamp a workpiece. The U end permits the strap to be completely removed when unclamped. The pin end is often used where space is limited or for clamping in horizontal holes.

Forged U clamp
Figure 8-21. Forged U clamp straps can be used for clamping at either the U end or the pin end.

Forged Adjustable Clamps. Forged adjustable clamps, Figure 8-22, are well suited where a heel support could interfere with the setup. These clamps are made as a complete unit and can accommodate a variety of workpiece heights. The steel pivot allows the fastening element to securely clamp at all elevations. This clamp is available in either forged steel or aluminum alloy.

Forged U clamp
Figure 8-22. Forged adjustable clamps have a built-in heel support.

Fastening Elements

The fastening element of a clamp strap is the device that actually applies force to the clamp. The two general types of fastening elements for strap clamps are threaded fasteners and cam handles. Threaded fasteners include a wide variety of bolts, studs, washers, nuts, and knobs.

One note of caution about fastening elements: always make sure that fasteners are specifically made for workholding operations. Standard hardware items are not strong enough for consistently safe clamping Likewise, many low cost, cut rate clamping components will not stand up to the repeated use required in workholding. These items often bend or rupture under the severe condition imposed on clamps in production applications.

Studs and Bolts. Two major types of threaded fasteners used for strap clamps are studs and bolts. Studs are the most common fastener for strap clamps. One end of the stud is usually mounted in a T nut; the other applies the holding force with a nut, as shown in Figure 8-23(a). Alternatives to the stud and T nut combination are T bolts, Figure 8-23(b), and T-slot bolts, Figure 8-23(c).

Forged U clamp
Figure 8-23. The three most common center fasteners for clamp straps are studs with T nuts, T bolts, and Tslot bolts.

Washers. Washers, a common item in many workholding applications, are also used with strap clamps. Figure 8-24 shows the common washers for fixturing. These include flat washers, C washers, swing C washers, knurled-face washers, and spherical washers.

The flat washer is one of the most common. With a clamp strap, the main purpose of the flat washer is as a shield between the clamp strap and the fastening element. This prevents any damage to the clamp strap when the fastener is tightened. When installed in a bolted assembly, flat washers are generally needed only under the nut.

The basic C washer is available in two styles, the plain and the swing C washer. These washers work well where a stud and nut, or bolt, clamp a part. With this washer, the nut or bolt is simply loosened a few turns so that the workpiece can be removed without completely removing the nut or bolt. The plain C washer is used where the washer must be completely removed from the assembly.

Flat washers, C washers, swing C washers, knurled-face washers, and spherical washers
Figure 8-24. Flat washers, C washers, swing C washers, knurled-face washers, and spherical washers are used for many workholding applications.

Figure 8-25(a) shows an application with a C washer installed in a groove in a drawbar arrangement. As the cam is rotated, the drawbar tightens against the C washer and clamps the workpiece. The swing C washer is designed for attachment to the workholder and is simply rotated, or swung, out of the way to load or unload parts, Figure 8-25(b).

Plain C washer
Figure 8-25. The plain C washer is used when the washer must be moved free of the assembly. The swing C washer is attached to the workholder and swung out of the way for loading/unloading.

Knurled-face washers are flat washers with a serrated face. In the example shown in Figure 8-26, the serrations grip two components of a fixture to prevent any sliding movement. Knurled-face washers are positioned both under the bolt head and nut. A close tolerance on the inside diameter of these washers prevents bolt slippage within the washer.

Knurled-face washers
Figure 8-26. Knurled-face washers are flat washers that have a serrated face to prevent any sliding movement.

Spherical washer sets are available in two different styles: the first is a set of washers with mating spherical faces, the second is a spherical washer and nut combination, Figure 8-27(a). Spherical washers act as a universal joint between the clamp and the stud or bolt. These washers reduce the strain and fatigue on threaded fasteners caused by repeated uses on workpieces of varying heights. Even slight differences in part heights can cause considerable fatigue in the fasteners. If left uncontrolled, the fatigue shortens the fastener life and will cause a safety hazard.

Spherical washer
Figure 8-27. Knurled-face washers are flat washers that have a serrated face to prevent any sliding movement.

The main reasons for fatigue are the variable height of the workpiece and the fixed height of the heel support. Since most parts have some height variation, the fastener is bowed and stressed each time it is clamped against a part. Spherical washer sets allow limited angular movement of the clamp strap with no effect on the fastener. The spherical joint eliminates the stress on the stud because it compensates for the angular misalignment of the clamp strap and fastener, Figure 8-27(b).

Cam Handles. Cam handles often act as the fastening element with strap clamps. Double-cam handles use an eyebolt arrangement, Figure 8-28. The eyebolt acts as the main mount for the double-cam handle, using a clevis pin.

The major benefit of cam-action clamps is the speed of operation. However, one word of caution: cam clamps rely on the friction between the cam lobe and the clamp or workpiece to maintain clamping force. Some operations with excessive vibration could cause a cam clamp to loosen, due to the inertial force of the handle.

Spherical washer
Figure 8-28. A double cam can be used with an eyebolt, instead of a stud and nut, on a slotted-heel clamp strap.

Nuts and Knobs. The nuts and knobs for clamp-strap assemblies are made in a variety of styles. Figure 8-29 shows the nuts most often found in these assemblies. They include the T nut, flange nut, spherical nut-and-washer set, hex nut, jam nut, knurled check nut, knurled nut, acorn nut, and coupling nut.

Spherical washer
Figure 8-29. A wide variety of standard and special purpose nuts are used for strap clamps.

The T nuts (a) are often used with flange nuts (b) or spherical nut-and-washer sets (c) in clamp strap assemblies. T nuts are mounted in the T slots of a machine table and anchor one end of the bolt or stud. The T nut is not threaded all the way through; rather, the threads stop one thread short of the bottom of the nut. This is done to prevent the stud from acting as a jack and breaking the T slot in the table. The flange nut or spherical nut-and-washer set is used on the other end of the stud. The flange nut, Figure 8-30, combines the advantages of a hex nut and flat washer into a single unit. The spherical nut-and-washer set goes even further by incorporating the advantages of the flange nut with a universal joint arrangement, as shown in Figure 8-31.

Spherical washer
Figure 8-30. Flange nuts are commonly used with studs and T nuts for strap clamps.
Spherical nut and washer sets
Figure 8-31. Spherical nut and washer sets compensate for workpiece height variations by permitting some angular movement of the clamp strap.

Hex nuts and jam nuts are for general purpose applications. With a first-class lever arrangement, a hex nut often sets the height of the fulcrum, Figure 8-32. Here the hex nut works with a jam nut (a) to fix the height of the fulcrum. Jam nuts can also be combined with spherical nuts and washer to lock the nuts in a fixed position (b). The jam nut is simply tightened against the other nut, locking both to the stud.

A knurled check nut (c) is often used for the same purpose, but since these nuts are knurled, they are intended for finger tightening. The knurled nut (d) is used when hand tightening is expected. The holes around these nuts provide for inserting a rod to apply additional clamping force when needed.

Jam nuts
Figure 8-32. Jam nuts are often used with first-class level arrangements to set the fulcrum height.

Two other nuts are frequently used with clamp straps. The acorn nut is a closed-end nut that protects the ends of studs. These nuts completely enclose the end of a stud to prevent damage to the threads, Figure 8-33. Coupling nuts act as a connector to join two or more studs, Figure 8-34.

Acorn nuts
Figure 8-33. Acorn nuts are closed-end nuts used for protecting the ends of studs, and preventing snags.
Coupling nuts
Figure 8-34. Coupling nuts are often used to connect two or more shorter studs to form one long stud.

In addition to the nuts, knobs are also used with clamp straps. Three common types of heavy-duty knobs are shown in Figure 8-35. They include the palm-grip knob, the hand knob, and the bar knob.

The palm-grip knobs (a) and hand knobs (b) are often used in place of a nut when more speed is needed and reduced holding force is acceptable. These knobs are available as tapped, reamed, or blank knobs, Figure 8-36. The bar knob (c) is used when more force is required. This knob is designed to be tightened with a bar inserted between the four prongs of the knob, Figure 8-37. Bar Handle nuts (Figure 8-38) are extension nuts with a sliding bar handle for excellent leverage. The bar can be moved to any position for convenient tightening, and is held in place by light spring force. Available in steel or stainless steel.

Knobs
Figure 8-35. Knobs are another element used to apply clamping force to strap clamps.
Hand knobs
Figure 8-36. Hand knobs are available in several styles, including tapped, reamed, and blank.
Bar knobs
Figure 8-37. Bar knobs can be turned easily by hand, then tightened by inserting a bar between the prongs.
Bar-handle nuts
Figure 8-38. Bar-handle nuts feature a sliding bar handle for extra leverage.

Heel Supports

The heel supports for clamp straps provide support at the end opposite the clamping point. The two basic types are threaded and block supports. Threaded supports are mounted either in the clamp strap or in the workholder base, depending on the design of the tool. Block supports can be custom made to suit the clamp height, or standard step blocks can be used.

Common heel supports for strap clamps include clamp rests, clamp rest screws, and stud leveling feet Figure 8-39.

Clamp rests, the most common, are used with the slotted-heel clamp straps. These supports are normally threaded into the tool body. Once set to the correct height, they are locked in place with a jam nut. The standard clamp rest (a) is made with a hex area below the contact. The miniature stainless type (b) is made with a hole, which allows turning with a rod. The clamp rest screw (c) is most often used for drill-spot-heel clamp straps. These clamp rest screws are also locked at the proper height with a jam nut. Stud leveling feet (d) are normally used with the tapped-heel clamp straps. Figure 8-40 shows how each of these heel supports is used.

Standard types of heel supports
Figure 8-39. Standard types of heel supports used with strap clamps.
Spherical nut and washer sets
Figure 8-40. Heel supports are selected to meet the requirements of both the clamping operation and the clamp strap.

Plain-end-type clamp straps most often use step blocks as heel supports. These blocks have a series of serrations, or steps, machined in their mating surfaces, Figure 8-41. The steps in one block engage an identical set of steps in the second block. This design allows the clamp strap to be positioned for different workpiece heights.

Plain-end-type clamp straps
Figure 8-41. Plain-end-type clamp straps often use step blocks as heel supports.

Other Clamp Strap Accessories

Other accessories for clamp straps are shown in Figure 8-42. They include guide blocks (a), clamp springs (b), and finger pins (c). Guide blocks are used as a rear slide block for tapped-heel-type clamp straps, Figure 8-43(a). These blocks position the clamping screw and reduce wear on the tool body. Clamp springs, Figure 8-43(b), are placed over the stud, between the clamp strap and the tool body. This spring makes loading and unloading the workholder easier because it keeps the clamp strap elevated off of the workpiece. The finger pins, Figure 8-43(c), are occasionally installed in clamp straps to aid sliding movement. Depending on the setup, finger pins can be used on either one or both sides of the clamp strap.

Other clamp strap accessories
Figure 8-42. Other clamp strap accessories.
Applications of guide blocks, clamp springs, and finger pins
Figure 8-43. Applications of guide blocks, clamp springs, and finger pins.