Thrust Bearings

Part of Gear Making

Bearings designed specifically to resist forces acting along the shaft axis, preventing axial movement while allowing rotation.

Why This Matters

Most people think of bearings as supporting the weight of a rotating shaft — that’s radial load. But many machines also push or pull along the shaft axis: propeller shafts are pushed forward by the propeller’s thrust, worm gear output shafts are pushed sideways by the helical tooth forces, millstone shafts carry the weight of the stone acting downward along a vertical axis, and water turbine shafts experience upward hydraulic thrust. Without thrust bearings, these axial forces cause the shaft to creep along its axis, eventually jamming machinery or pulling shafts out of their radial bearings.

The problem becomes acute in gear-driven systems. Helical gears, worm gears, and bevel gears all produce significant axial thrust forces as a byproduct of their geometry. A gearbox without proper thrust bearings will have shafts walking out of alignment within hours of operation, destroying everything.

Thrust bearings are also one of the most improvable bearing types — many designs can be fabricated from simple materials without precision metalworking equipment, making them important for post-collapse machine building.

Types of Thrust Bearings

Step bearings (also called footstep bearings) are the oldest and simplest type: the shaft’s lower end rests on a hardened pad. The entire axial load is borne by the flat contact between shaft end and pad. Used on vertical shafts in mills since antiquity, a bronze pad under a vertical millstone shaft is a classic application. The shaft rotates on the pad; the pad sits in a housing that can be adjusted for height. Lubricated with oil or grease in a cup around the pad. Effective for slow-speed, very heavy loads.

Collar thrust bearings use one or more collars machined onto the shaft that butt against stationary bearing surfaces. As the shaft tries to move axially, the collar face presses against the bearing face. The bearing surface material is typically babbitt or bronze. Multiple collars (multiple collar thrust bearing) spread the load over larger area, allowing more thrust capacity in the same shaft length.

Ball thrust bearings use balls arranged in a flat cage between two hardened washers (races). One washer is fixed to the housing; the other rotates with the shaft. Load is transmitted through the balls in compression. Very low friction, handles high thrust loads in compact space. Can be salvaged from automotive wheel bearings and steering columns. Balls operate in pure thrust (no radial) — they’re not designed for radial loads and will fail quickly if both are applied simultaneously.

Tapered roller thrust bearings are the most capable type for combined radial and axial loads. Used in vehicle differentials, machine tool spindles, and gear reducers. Salvaged from automotive differentials, these are among the most useful bearings to recover from collapsed-civilization machinery.

Pivot bearings: A conical or spherical shaft end resting in a matching conical or spherical cup. Used for precision instruments and slow-speed light-load applications. The watch industry used gem pivot bearings (ruby cups) for centuries. At the other extreme, a steel ball pressed into the shaft end resting in a steel cup provides a low-friction pivot for lightly loaded vertical shafts.

Designing a Collar Thrust Bearing

For a typical gearbox output shaft application, a single-collar thrust bearing is straightforward to fabricate:

Sizing the collar: The collar outside diameter should be large enough that the contact stress on the babbitt face doesn’t exceed the material’s limit. Babbitt can handle about 400-500 psi thrust load. If the axial force is 1,000 lbs and the shaft is 2 inches diameter:

Contact area needed = 1,000 ÷ 400 = 2.5 square inches Collar OD² = (4 × 2.5 ÷ π) + shaft ID² = 3.18 + 4 = 7.18 Collar OD = 2.68 inches

Round up to 3 inches for safety margin.

Construction:

1. Machine the collar on the shaft (turn a shoulder, or press-fit a separate collar ring onto the shaft and secure with a key)
2. Cast or machine the thrust bearing housing — a bronze or babbitt-lined housing with a recessed face matching the collar
3. The housing mounts fixed to the machine frame; the shaft collar rotates against the babbitt face
4. Oil groove: cut a radial groove in the babbitt face to distribute oil from the center to the periphery
5. Oil hole: drill a hole from the housing exterior to the center of the babbitt face; fit an oil cup

Collar clearance: The collar should have about 0.003-0.005 inches axial clearance between it and the thrust faces on each side. This allows an oil film to form. Too tight and it runs metal-on-metal; too loose and the shaft bounces axially under varying load.

Fabricating a Step Bearing

For a vertical shaft application (millstone spindle, vertical turbine):

1. Bearing pad material: Bronze or hard cast iron works best. Cut a disc slightly larger than the shaft end diameter. Drill a shallow oil cup in the center. Polish the contact surface with fine abrasive.

2. Pad housing: A cast iron cup that holds the pad and is adjustable for height (threaded adjustment, or shim stack). The housing must be securely anchored to the floor or frame — it carries the full weight of the shaft and everything on it.

3. End of shaft: Machine the shaft end flat and smooth. A slight crown (convex radius of a few inches) prevents the sharp corner from digging into the pad when the shaft is slightly off-vertical. Harden the shaft end by local heating and quenching, or press a hardened steel ball bearing into the shaft end.

4. Lubrication: An oil cup around the pad, fed by a wick or manual oiling. On very heavily loaded step bearings, forced circulation oil may be needed — a hand-pump squirts oil into the bearing daily.

For a millstone: The step bearing carries the full weight of the runner stone (which can be 500-2,000 lbs) plus any additional downward forces. The bearing pad needs to be large enough, made of hard material, and checked regularly for wear. Historical mills often used a hard wood (like lignum vitae) step bearing for heavy millstones with good results — wood conforms slightly to imperfect shaft ends and tolerates grit and water in the lubrication.

Installation and Maintenance

Thrust bearing preload: Some applications require the bearing to be slightly preloaded (loaded even before external thrust is applied) to maintain rigidity. Adjust with shims until the shaft moves axially only under intentional force, not under vibration.

Wear monitoring: Axial play in the shaft increases as thrust bearing faces wear. Check by pushing the shaft firmly in each direction and measuring total axial movement with a dial indicator. More than 0.010 inches in most applications warrants inspection and possible rebabbitting.

Overheating: Thrust bearings overheat even faster than radial bearings under lubrication failure because the load is concentrated on a smaller area. If a thrust bearing housing is too hot to touch after 10 minutes of running, shut down immediately.

End float: On machines with both radial and thrust bearings, only one thrust bearing should be preloaded or fixed — the other should be able to move slightly to accommodate thermal expansion of the shaft. Constraining a shaft axially at both ends sets up enormous stresses as it heats up.