EngineeringMechanical EngineeringPart 10 of 15
Shafts, Keys, Circlips and Seals
The shaft carries the torque, the key transmits it, the circlip locates the assembly and the seal keeps the lubricant in. Four small decisions that between them determine whether the drive train works.
- Standard shaft and key sizes
- Key stress rules
- Circlip thrust limits
- Seal selection
Executive summary
Availability governs shaft design more than analysis does. Bright steel shafting is stocked in a standard series of metric diameters, each with an associated standard key size, and design should start by choosing from that list rather than by calculating a diameter and hoping to buy it.
Around the shaft sit three families of small components that fail disproportionately often: keys, which are usually checked for shear but fail in bearing; circlips, whose thrust rating is usually governed by the groove rather than the clip; and seals, which are selected for the shaft surface as much as for the fluid.
Standard shafts and keys
Common bright steel diameters run from 8 mm up to about 120 mm in general stock, with larger sizes to around 400 mm available on order. The key width and height are standardised against the shaft diameter, and shaft tolerance widens in steps as diameter increases.
| Shaft diameter (mm) | Key width × height (mm) | Typical shaft tolerance (mm) |
|---|---|---|
| 8, 10, 12 | 2×2, 3×3, 4×4 | +0 −0.08 |
| 15, 16, 20, 22 | 5×5, 5×5, 6×6, 6×6 | |
| 25, 27, 30 | 8×7 | +0 −0.10 |
| 33, 35, 39, 40 | 10×8, 10×8, 12×8, 12×8 | |
| 45, 50 | 14×9 | +0 −0.12 |
| 55 | 16×10 | |
| 60, 65 | 18×11 | |
| 70, 75, 80, 90, 100 | 20×12, 20×12, 22×14, 25×14, 28×16 | |
| 110, 120 | 28×16 and above | +0 −0.15 |
Keyway tolerance is separate from shaft tolerance and depends on the class of fit required: free, normal or close (interference). Specify the class explicitly; leaving it to the machinist produces assemblies that either rattle or cannot be dismantled.
Shaft materials
Plain carbon steel
Grades 1020, 1030 and 1040 or 1045. The general-purpose choice. Higher carbon content gives higher strength and better wear resistance where a journal runs directly on the shaft.
Stainless steel
Austenitic 304 and 316 for corrosion resistance where strength demands are modest; martensitic 420 and 431 where hardness and strength matter as well.
Alloy steel
Grades 4140 and 4340 for high strength shafting, heat treatable to substantially higher yield than plain carbon grades.
Hot rolled round bar has a commercial surface finish and dimensional tolerance. It is not suitable for rotating shafts running in bearings or seals. Use bright steel for shafts and keep hot rolled sections for structure.
Key and keyway stresses
A key is loaded in two ways at once: shear across its width, and bearing on the flank that transmits the torque. Both must be checked, and it is usually the bearing stress that governs in a well proportioned assembly.
- Applies to
- Both the key itself and the shaft or hub material, whichever is weaker.
- Status
- Design rules of thumb for preliminary sizing. Use the governing standard for critical assemblies.
| Check | Stress area | Comment |
|---|---|---|
| Shear | Key width × key length | Compare against the allowable shear stress of the weaker material. |
| Bearing on key | Half key height × key length | Only part of the key height bears; the remainder sits in the hub keyway. |
| Bearing on shaft or hub | Keyway depth × key length | Base the check on the weaker of key, shaft and hub material. |
Where a single key is inadequate, the options in order of preference are a longer key, a larger shaft diameter with the associated larger standard key, two keys at 180 degrees, or a splined connection. Two keys should be treated as sharing the load imperfectly unless machining accuracy justifies otherwise.
Circlips and retaining rings
Bore circlips
Fit a groove in a housing bore to retain a bearing or bush. Lugs with small holes take circlip pliers for assembly and removal.
Shaft circlips
Fit a groove on a shaft. Also lugged for pliers, and the most common general-purpose retaining device.
Push-on external
Applied radially from the side of the shaft rather than expanded over the end. Fast to fit, lower thrust capacity.
No-groove types
Grip the plain shaft or bore directly. Not intended for repetitive disassembly and cannot carry high thrust loads.
Materials and finishes
Section shapes are commonly rectangular, square or round. The standard material is high carbon spring steel with a phosphate and oil finish; zinc, cadmium, nickel and tin plating are all offered. Stainless steel and phosphor bronze are the usual alternatives where galvanic corrosion resistance is required — and where they are used, the galvanic compatibility of clip and groove material must be checked, because the pair is a couple whether or not it was designed as one.
Thrust rating: two numbers, not one
- T1
- maximum safe thrust load on the circlip itself
- T2
- maximum safe thrust load on the groove in the shaft or bore
- Correction
- T2 is quoted for a nominated shaft yield strength. For other shaft materials scale it by the ratio of actual yield to the reference yield.
Published thrust figures assume pure shear in the clip, a sharp-cornered abutting part that is a slide fit, standard clip material and steady loading. Unless the shaft is made from a high yield material, the limiting value is almost always the groove strength rather than the clip strength. A radiused or chamfered abutting component invalidates the assumption entirely and tilts the clip out of its groove.
Rotary shaft seals
A lip seal is selected against four things: the shaft, the housing bore, the fluid and the operating conditions. Three of those are decided by other people's design decisions, which is why sealing problems usually surface late.
Seal selection determines a shaft surface specification, a bore specification and a lead-in chamfer. All three are cheap to include on the first drawing and expensive to add after the shaft is machined.
Design checklist
- Shaft diameter selected from the standard bright steel series rather than from calculation alone.
- Shaft material grade specified, with hot rolled bar excluded from rotating applications.
- Key size taken from the standard series for the shaft diameter.
- Key checked in both shear and bearing against the weakest of key, shaft and hub.
- Keyway fit class stated explicitly as free, normal or close.
- Circlip type chosen for the assembly and disassembly method actually intended.
- Thrust rating taken as the lower of clip and groove values, with the groove figure corrected for shaft yield.
- Abutting components confirmed sharp-cornered and a slide fit where the thrust rating assumes it.
- Galvanic compatibility of clip and groove materials checked where special materials are used.
- Seal shaft surface finish, hardness and lead-free requirement specified on the drawing.
- Housing bore, depth and lead-in chamfer detailed to suit the seal.