Press fits, splines and retainers

Not everything is bolted or welded. A great deal of a drivetrain is held together by nothing more than metal squeezing metal, teeth meshing with teeth, and a sprung wire sitting in a groove. Each of those three has a capacity, and each has a way of being asked to do more than it can.

A hydraulic arbor press driving a ball bearing onto a splined steel shaft on a workbench, with the press gauge in view, a chilled shaft in an insulated tray of dry ice beside it and a set of retaining rings and a circlip plier laid out on the bench. — A bearing going on under load. What holds it there is nothing but the pressure between two pieces of steel.

Interference fits — friction, and nothing else

Press a shaft that is larger than its hole into a hub and both parts deform elastically: the shaft is compressed, the hub stretched. The resulting radial contact pressure, multiplied by the contact area and the friction coefficient, is the entire holding capacity.

A diagram of interference fit mechanics. A shaft and hub are shown in section before assembly with the interference exaggerated and labelled, then after assembly with the shaft compressed, the hub stretched and radial contact pressure arrows distributed around the interface. A formula-free annotation indicates that holding torque comes from pressure times area times friction. Beside it a bar comparison shows the effective interference achieved by three methods, press fitting with the surface asperities shown sheared and flattened and the bar shortest, shrink fitting with the hub heated and the bar full height, and cryogenic fitting with the shaft chilled and the bar full height. A third panel shows a steel shaft in an aluminium hub at room temperature and at operating temperature, with the aluminium bore having grown more than the shaft and the remaining interference marked as much reduced. A fourth small panel shows a lead-in chamfer and pilot diameter on the shaft nose, contrasted with a sharp edge shown scoring the bore and packing swarf ahead of it. — There is no key and no bond. Anything that reduces the interference — smeared asperities, a hot aluminium hub — reduces the capacity in direct proportion.

Two consequences follow immediately.

Pressing is worse than a thermal fit for the same interference. Sliding the surfaces past each other under pressure shears and smears the asperities, so part of the measured interference is consumed before the joint is complete. Heating the hub or chilling the shaft lets the parts meet without rubbing, and the full interference develops.

Method	How	When chosen
Press fit	Force, at room temperature	Simple, quick, moderate duty
Shrink fit	Heat the hub, drop the shaft in	Higher capacity, larger parts
Cryogenic fit	Chill the shaft, let it expand	Where heat would damage a coating, bearing or heat treatment

And a dissimilar material fit has to be calculated hot. Aluminium expands about twice as much as steel, so a steel shaft in an aluminium hub loses interference as the assembly warms up.

The designation that specifies all this — H7/s6 and its relatives — is set out in fits, limits and clearances. The capital letter is always the hole, lower case the shaft, the letter fixes where the tolerance zone sits and the number how wide it is.

Splines and keyways

Both transmit torque between a shaft and a hub. They are not interchangeable in the way the drawings sometimes imply.

A comparison of a keyway and a spline. On the left, a shaft with a single rectangular key in a keyway is shown in cross section with the whole torque reacted at one location, a stress concentration marked at the keyway corners, and the necessary assembly clearance drawn at the key flanks. An adjacent sketch shows a reversing torque hammering across that clearance and the keyway edges beaten and burred. On the right, an involute spline is shown in cross section with the torque shared across many teeth around the full circumference, the load arrows distributed evenly and the shaft remaining balanced. Beneath, two further sketches distinguish a fixed spline, fully engaged and located, from a flexible spline shown sliding axially with a gearbox input shaft, with lubrication indicated between the teeth. — One key reacts everything at a single point and needs clearance to be assembled. Under a reversing load that clearance is the beginning of the failure.

	Single key	Spline
Load sharing	One location	Many teeth, full circumference
Stress concentration	High, at the keyway corners	Much lower
Reversing torque	Hammers across the clearance	Tolerated
Balance	Asymmetric	Symmetric
Cost	Low	Higher

A key needs clearance on its flanks to be assembled at all. Under a reversing load the hub hammers back and forth across that clearance, and the keyway edges are progressively beaten out until there is play, then noise, then failure. A spline shares the torque across many teeth and stays symmetric, so the shaft is balanced and the bending stress lower.

Splines come in two functional flavours, and the distinction is not about quality:

A fixed spline locates as well as drives — it must not move.
A flexible spline is deliberately a sliding fit, because something has to move: a gearbox input shaft under clutch engagement, a propshaft accommodating suspension travel.

The tolerance, the surface treatment and the lubrication all follow from which of the two it is. A sliding spline that was toleranced as a fixed one wears out; a fixed one toleranced as a sliding one rattles.

Retaining rings — and what they are not for

A circlip sits in a shallow groove and reacts against the groove wall. It takes a modest axial thrust, and that is all.

So the rule is simple: a retaining ring retains, it does not carry working load. Where there is real axial force, the load path is a machined shoulder, a nut, or a flange — and the clip, if it is there at all, is keeping something in place during handling.

Choosing between the three

Requirement	Reach for
Torque, one direction, low cost	Keyway
Torque, reversing or high	Spline
Torque with no relative movement, compact	Interference fit
Axial location only, incidental thrust	Retaining ring
Axial location under real load	Shoulder and nut

Interference fits and splines are often combined — a splined shaft with an interference-fitted bearing beside it — because they are answering different questions. The spline handles torque, the press fit handles position, and the two capacities are calculated separately.

What to take away

An interference fit holds by friction from contact pressure. No key, no bond.
Pressing smears the asperities, so it holds less than a thermal fit of the same interference.
Shrink and cryogenic fitting develop the full interference without rubbing.
A steel-in-aluminium fit loses interference when hot — calculate it at temperature.
A lead-in chamfer is what stops a press fit scoring and seizing.
A single key concentrates torque at one point and is hammered out by reversing loads.
A spline shares load across many teeth and stays balanced.
Fixed and flexible splines are different functions with different tolerances.
A retaining ring retains; it is not a load path.
The groove, not the ring, usually sets the capacity — and it is a fatigue notch.