Choose a Spieth clamping set when the joint must run backlash-free, hold concentricity without cutting a keyway into the shaft, carry torque and axial load together, or come apart and reassemble repeatedly. Choose a keyway when duty is slow, lightly loaded and cost-driven. The deciding factor is usually backlash, not torque.
What a Spieth clamping set does differently
Spieth machine elements are one-piece, rotationally symmetrical parts that grip through elastic deformation. Axial force, applied with integrated clamping screws, is turned by the element’s geometry into a radial clamping force spread evenly around the full circumference. The manufacturer draws the contrast directly: a large number of conventional shaft-hub solutions initiate force at individual points, while the Spieth design applies it all round.
Two consequences matter to a designer. First, because the element is one piece and symmetrical, it centres itself, so runout and balance stay predictable on anything that rotates. Second, because it grips by friction over its whole circumference instead of through a key, there is no groove and no slot cut into the shaft — and no clearance to take up. Spieth describes the resulting connections as backlash-free, able to be released and re-used at the same position, and machined to tolerances in the thousandths of a millimetre.
The principle goes back to the company’s founder, Rudolf Spieth, and has been refined for more than sixty years. The same geometry carries across the range — locknuts, guide bushings and clamping nuts as well as clamping sets — which is why the shaft-hub versions share the centring and backlash behaviour of the nuts.
Where a keyway still wins
It would be a mistake to present keyed joints as obsolete. A keyway is a sound engineering answer in plenty of situations, and telling a customer otherwise is how a supplier loses credibility.
- Cost and availability. A standard key, a broached hub and a milled shaft are cheap, and any machine shop can produce them. A precision clamping element is a purchased part with a lead time attached.
- Slow, lightly loaded duty. A pulley on a low-speed fan shaft, a handwheel, or a sprocket on a light conveyor does not need a backlash-free joint. Nothing in the application notices the clearance.
- Tolerance to looser fits. A keyed joint accepts looser fits and a rougher surface finish than a clamping set, which is specified against h5/h6 — or k6/m6 on motor shafts. Where the shaft is already made, that difference decides the question.
- Field repair. A replacement keyway can be cut on site by a local shop instead of waiting for a specialist element.
- Universally understood. Every fitter knows how a keyed joint goes together, which matters on equipment maintained in the field.
The limitations are just as well established. A keyway transmits torque only; on its own it locates nothing axially. It requires clearance to assemble, and that clearance becomes lost motion each time the drive reverses. And it is cut on one side, so the joint is not rotationally symmetrical.
The five points that decide it
Work through these in order. The first two decide most cases, and the answer is usually clear before you reach the fifth.
- Backlash and repeatability of position. A keyed joint must have clearance to slide together, so every reversal spends that clearance before anything moves. On an indexing table, a reversing servo axis or a measuring axis, that lost motion lands in the part. Clamping sets are specified as backlash-free, which is why they appear on this kind of duty.
- Concentricity and balance. A keyway is an asymmetric cut, so concentricity depends on how well the fit happens to come out, and a rotating joint carries an imbalance. A clamping set is rotationally symmetrical and self-centring, so runout is a property of the element rather than of the assembly luck.
- Shaft section. A keyway removes material from the shaft and leaves a stress concentration at the corner of the slot — the reason keyed shafts sometimes fail at the keyway corner rather than at the load. A clamping set grips the outside of the shaft and cuts nothing. Be precise about the trade, though: the hub has to carry the radial clamping pressure, so a thin-walled hub is the part to check on a keyless joint.
- Torque and axial load in one joint. A keyway carries torque and leaves axial location to a shoulder, a circlip or a retaining nut. A clamping set resists both through the same friction joint, which removes parts from the design and shortens the stack.
- Serviceability and repeat assembly. Removing a keyed joint means driving the key out and often pressing the parts apart, and each cycle wears the fit. A Spieth clamping set releases by slackening its clamping screws and can be brought back to the same position — the manufacturer’s point about releasing and re-using a connection. On equipment that is stripped for maintenance or changed over between products, that difference shows up in downtime.
Keyway vs Spieth clamping set, side by side
| Point | Keyed connection (keyway) | Spieth clamping set |
|---|---|---|
| Load path | Key flanks, at localised points | Friction distributed over the full circumference |
| Backlash | Clearance needed to assemble; lost motion on reversal | Backlash-free by design |
| Concentricity and balance | Asymmetric cut; runout depends on the fit achieved | Rotationally symmetrical, self-centring |
| Shaft | Keyway removes section and concentrates stress | No groove or slot cut into the shaft |
| Axial location | Torque only — needs a shoulder, circlip or nut | Torque and axial force in the same joint |
| Assembly and removal | Key driven out, often pressed apart; fit wears on repeat work | Released by slackening the screws; re-usable in position |
| Speed | Clearance and asymmetry work against high-speed duty | Suited to high-speed, high-precision duty |
| Fit required | Tolerates looser fits and rougher surfaces | Specified against h5/h6, or k6/m6 on motor shafts |
| Cost | Low; standard and locally machinable | Higher — a purchased precision element |
| Where it fits | Slow, lightly loaded, cost-driven, rarely adjusted | Precision spindles, servo axes, test rigs, frequent changeover |
Which one to choose
Choose a Spieth clamping set when any of the following is true: the axis reverses or indexes and backlash would show in the work; the joint rotates fast enough for balance to matter; a keyway would remove too much section from a small shaft; the design needs axial location without adding a shoulder or nut; or the joint is taken apart for maintenance or changeover.
Choose a keyway when the duty is slow and lightly loaded, the tolerances are loose, the joint is assembled once and left alone, and cost is the binding constraint. A one-off bracket, a low-speed pulley or a handwheel does not justify a precision element.
The mixed case is the common one. There is no rule that says a machine has to be consistent. Keeping a keyed joint on a low-speed fan pulley while fitting a clamping set to the same machine’s servo-driven ball screw is normal engineering — the two joints are being asked different questions. Decide per joint, on backlash and concentricity, not per machine.
One practical caveat on retrofits: a clamping set is specified against a fit, so a shaft that has already been turned to a loose keyed fit may not be usable as-is. Measure before committing.
The four Spieth clamping set series
Spieth splits its clamping sets by fit and by duty rather than by size alone. That is the fastest way to narrow the choice.
| Series | Fit | Construction | Typical duty |
|---|---|---|---|
| DSK / DSL | h5 / h6 | Positive-lock shaft-hub connection with integrated clamping screws; DSK is the short version, DSL the long version with higher transmittable forces | General precision joints needing high concentricity |
| DSM | k6 / m6 | Compact precision clamping set, sized for motor shafts | Motor and gearbox shafts under dynamic load |
| AK / IK | h5 / h6 | Friction-fit connection with free force transmission, self-centring | General machine and plant engineering, drive duty |
| AL / IL | h5 / h6 | Tapered clamping set giving uniform contact pressure | High-precision work at high speeds |
Sizing is written into the part number. A DSK 14.26 is the 14 mm shaft diameter paired with a 26 mm bore; the second number is the outside diameter, so the same shaft can be paired with different bores depending on the hub wall you can afford. Across the published DSK/DSL table the shaft side starts at 14 mm, and the clamping screws run from M3 up to M8 to ISO 4762.
Two further points on selection. The axial space decides DSK against DSL more often than the torque figure does — DSK is the short element and DSL the long one, and the long version carries the higher forces. And the fit is not negotiable: sending a shaft made to a loose keyed fit for a clamping set is the most common way these projects lose a week.
Sizing a Spieth element — what to send
To specify one, five things are enough:
- Shaft diameter (d1) and hub bore (d2), measured rather than assumed.
- The torque to transmit, and any separate axial force.
- Whether the fit is h5/h6 or k6/m6 — this selects between DSM and the rest.
- The axial space available along the shaft.
- Shaft speed and duty cycle, if the joint rotates continuously.
With those we check the series against the manufacturer’s tables and come back with a size, rather than guessing at one.
- See the Spieth range or browse all brands.
- Clamping sets sit in the drive train — see motors, gearboxes and transmission and the full product range.
- More selection guides are listed under technical articles.
- Send the shaft and bore figures through the inquiry form.
Sizes and specifications in this article follow the manufacturer’s published data at the time of writing. Confirm the current table before ordering.