A correctly sized shaft collar starts with the bore fit, not the OD: the collar's ID must match the shaft to a precision clearance/transition band, while the OD, width, material, and fastener strategy follow the load case [S2]. Get the bore wrong and nothing else matters — the collar slips, the shaft frets, or the assembly never reaches its rated torque.
Specifying a collar is a four-axis decision: bore diameter and tolerance class, body style (1-piece solid, 2-piece clamp, single-split), material and finish (zinc-plated steel, black-oxide steel, 303/304/316 stainless, 2024/6061 aluminum), and fastener type (set screw, socket-head cap screw, lever). Each axis constrains the next, and the load case — axial stop, radial limit, spacer, or sensor/encoder mount — picks the combination.
Bore Sizing and Tolerance: Where the Selection Actually Starts
Standard collar bores from Chinese OEM catalogs are listed in 1 mm increments from roughly 3 mm through 100 mm, with inch-series (1/4" through 3" and beyond) running in parallel for US-built machinery [S2]. Specifying a bore 0.001"–0.002" (≈0.025–0.05 mm) under nominal shaft size with an interference or light-transition fit is the universal baseline; loose slip fits invite fretting wear and torque loss within weeks of duty cycle [S2].
For high-precision drives — servo, encoder mounting, ball-screw supports — collars are routinely ordered to g6 (shaft) / H7 (bore) tolerance, with the shaft ground and the collar OD turned concentric to ≤0.05 mm TIR. For conveyor idlers, agricultural shafts, and general industrial drive trains, a clearance fit (H7/h6 or H8/h7) is acceptable as long as the set screw or clamp grip carries the torque, never the friction of the bore. Material suppliers note the bore tolerance carries through to the OD concentricity spec — a sloppy bore propagates into runout at the sensor face [S2].
One-Piece vs Two-Piece Clamp vs Single-Split: Style Choice by Service
One-piece (solid) collars are the cheapest and strongest per gram but must be installed end-of-shaft before the next component — a hard constraint in any retrofit or field-service scenario. Two-piece clamp collars wrap the shaft with a through-bolt or socket-head fastener, allowing installation anywhere along the shaft without disassembly, and the clamping action keeps the shaft mar-free [S2]. Single-split collars (a 1-piece body with a single axial saw-cut) split the difference: cheaper than full 2-piece, installable mid-shaft, but with lower holding torque than a 2-piece clamp [S2].
Holding-torque hierarchy runs roughly: 2-piece clamp > 1-piece solid with dual set screws > single-split with single screw > 1-piece solid with one set screw. The clamping force of a 2-piece collar is set by the cap-screw preload, so fastener grade (8.8, 10.9, 12.9) and thread size — typically M3 through M12 for small-bore collars — directly govern how much torque the collar can transmit before slipping. For reversing drives, vibratory feeders, and pump shafts, spec the 2-piece clamp; for fixed-orientation motor mounts and conveyor idlers where the shaft has a free end, the 1-piece solid is the rational pick [S2].
Material and Finish: Steel, Stainless, Aluminum by Environment
Stock catalog material options in 2024–2026 commercial offerings cover four families: zinc-plated carbon steel (the budget default for indoor dry service), black-oxide carbon steel (mild corrosion resistance, good appearance for visible machinery), 303/304/316 stainless steel (food, washdown, marine, chemical exposure), and 2024-T3/6061-T6 aluminum (≈1/3 the density of steel, for weight-critical or non-magnetic applications) [S2]. The full stock matrix — zinc steel, black-oxide steel, stainless steel, aluminum — is published across multiple ISO 9001-certified Chinese OEM catalogs and matches US/EU distributor lines.
316 stainless carries the best chloride and acid resistance of the common collar grades and is the default for chemical-plant, pharmaceutical, and offshore duty; 303 is free-machining but its sulfur content caps its corrosion resistance below 304/316 and rules it out for many acid exposures. Aluminum collars are common on motion-control stages where magnetic signature must be near zero or where rotating mass at the shaft end has to be minimized. The trade-off is hardness: a 6061-T6 collar will deform under a set screw long before a steel one does, so aluminum always pairs with a hardened set-screw tip or a clamp-style design. Shaft keys and collars are sometimes confused, but they solve different problems — a key transmits torque through a keyed profile, a collar transmits torque through friction or by acting as a fixed axial stop.
Set-Screw, Clamp, and Lever: Fastener Strategy by Load
Set-screw collars (typically two 1/4-28 or M3–M6 cup-point or knurled-tip screws on a 1-piece body) carry modest torque and are the lowest-cost option. Holding torque scales with screw size and quantity: a single #10-32 set screw on a 1" bore might hold 15–25 N·m before slip; dual #10-32 screws push that to 50–80 N·m; a full 2-piece clamp with an M6 socket-head cap screw at 8.8 grade can hold several hundred N·m on a mid-size bore [S2].
Clamp-style 2-piece collars eliminate the bore-damage risk of set screws and are specified wherever the shaft is hardened, ground, or otherwise expensive to refurbish. Lever collars (quick-release) are a specialty item for jigs, fixtures, and test stands where the position must be adjusted by hand without tools. For paired components on the same shaft — a collar plus a shaft coupling, or a collar plus a linear guide carriage stop — the design rule is to lock positions first with the collar, then assemble the rest of the stack against that fixed reference.
Load Cases: Axial Stop, Radial Limit, Spacer, Mounting Platform
Four load cases dominate. (1) Axial stop — the collar prevents a bearing or gear from migrating along the shaft; high axial load, low torque, and a retaining-ring or shoulder can do the same job at lower cost. (2) Radial limit — the collar acts as a mechanical stop for a lever, cam, or limit-switch actuator; shock loading applies, so a 2-piece clamp is safer. (3) Spacer — the collar sets axial spacing between two components; it carries no load beyond its own weight and is the lowest-stress use case. (4) Mounting platform — a collar with a tapped face (M3–M6 holes on the OD) supports an encoder, bracket, or sensor; the load case is the cantilever weight of the mounted component plus its inertia. [S2]
For the mounting-platform case, OD flatness and face runout are the controlling specs — a 0.1 mm face runout on a 50 mm OD collar can read as 0.05° angular error on a coupled encoder, enough to break position-loop bandwidth on a high-resolution servo. Material guidance for harsh service is covered separately in chemical-duty collar selection, which lines up 303 vs 304 vs 316 and aluminum against pH, chloride, and temperature envelopes.
Selection Checklist: Bore, Style, Material, Fastener, Load
A working shortlist runs in this order: (1) bore = nominal shaft size, tolerance = g6/H7 for precision, H8/h7 for general; (2) style = 1-piece solid if shaft end is free, 2-piece clamp if mid-shaft, single-split if budget-constrained mid-shaft; (3) material = zinc-plated steel indoors, 304 stainless in washdown, 316 stainless for chemical/marine, aluminum for weight or non-magnetic; (4) fastener = single set screw for light duty, dual set screws for moderate, 2-piece clamp bolt for reversing or high-torque, lever for jigs; (5) load = confirm axial stop, radial limit, spacer, or mount-platform case and size the collar body width accordingly. Width is the unsung parameter: a 1/2" wide collar on a 1" shaft is roughly half the axial load capacity of a 1" wide one, and doubling width roughly doubles axial rating without changing OD. [S2]
For paired shaft keys and collars on the same shaft, lock positions with the collar first, then key the mating component — the collar's slip-prevention is what keeps the keyed joint from back-driving the key. On new builds, order one extra collar per shaft as a field-replacement spare: a 2024-vintage ISO 9001 catalog lists single-split collars in zinc steel, black-oxide steel, stainless, and aluminum as stocked SKUs with short lead times [S2].