A flat thrust bearing is two hardened, polished steel washers, one fixed and one turning with the shaft, with a bronze washer or a ring of hardened balls between them to carry the axial load. It suits moderate loads at low speed, and its limit depends on keeping oil at the washer faces and on the rpm, because the sliding speed climbs towards the rim of the washer.
Part of Bearings and Bushings
The simplest bearing for load along a shaft: flat rings stacked face to face, with a soft or rolling layer between them.
Axial Load Needs Its Own Bearing
A sleeve bearing carries load across the shaft. Many machines also push or pull along it: a screw press, a worm gear, a vertical shaft carrying a stone or a turbine. A sleeve cannot carry that load, so the shaft needs a face that takes it. The simplest is a flat ring on the shaft pressing on a flat ring in the housing, with something between them that is meant to wear. Thrust Bearings gives the wider range of types; this article is the flat one.
The carrier article specifies the design: two hardened, polished steel washers with a bronze washer or a hardened ball set between them, adequate for moderate loads and low speeds.
What You Need
- Two steel washers, hardened and tempered (Hardening, Tempering), with faces polished flat
- A bronze washer to run between them, or hardened balls in pockets (see the captured-ball design in Roller Bearings)
- A housing that holds the fixed washer square to the shaft
- Oil or grease and a way to keep it at the faces (Lubrication Systems)
Method: Build And Fit A Flat Thrust Bearing
Step 1: Make and harden the steel washers.
Step 2: Flatten and polish the faces. Check them for flatness with a straight edge or with blue (Testing Flatness). A face that is not flat carries its load on a high spot.
Step 3: Cut radial oil grooves in the bronze washer so oil flows from the centre to the rim.
Step 4: Fit the stack with the fixed washer square in the housing and the turning washer fixed to the shaft. The faces must be perpendicular to the shaft axis; if they are not, the load rides on one edge.
Step 5: Lubricate and check axial play by pushing the shaft along its axis.
Worked Example
A washer with 100 mm outside and 60 mm inside diameter carries 3,000 N axial load at 100 rpm. The load and the dimensions are chosen for the example.
- Contact area = 3.1416 x (0.05 x 0.05 - 0.03 x 0.03) = 3.1416 x 0.0016 = 0.00503 square metres
- Face pressure = 3,000 / 0.00503 = 597,000 Pa, about 0.6 MPa
- Mean radius = (50 + 30) / 2 = 40 mm
- Friction torque with a lubricated bronze-on-steel coefficient of 0.02-0.06 = 0.02 x 3,000 x 0.04 = 2.4 N m up to 0.06 x 3,000 x 0.04 = 7.2 N m
- Angular speed at 100 rpm = 2 x 3.1416 x 100 / 60 = 10.47 rad/s
- Heat = torque x angular speed = 25 W up to 75 W
At 300 rpm the same bearing makes three times the heat, 75-226 W. The coefficient used is the lubricated one, so the figures assume oil at the faces. Dry, at 0.10-0.20, the same bearing at 100 rpm makes 126-251 W (0.10 x 3,000 x 0.04 x 10.47 up to 0.20 x 3,000 x 0.04 x 10.47) and fails. No allowed face pressure for bronze is given in this wiki. Run the bearing under reduced load first and watch the temperature rise.
Common Mistakes
- Letting the faces run dry. Oil must reach the rim, where the sliding speed is highest.
- Fitting the washers out of square. The load concentrates on one edge.
- Using soft steel for the washers. They dent and score; they must be hardened.
- Quoting a load for a thrust bearing without a speed or lubrication. The number depends on both.
Safety
A load on a thrust bearing can drop if the bearing fails
A vertical shaft carrying a stone, wheel or turbine rests on the bearing. Before inspecting it, block the shaft so the load cannot fall, and stop the drive so nothing turns while your hands are near the faces. Name the stop and its coast-down time.
What’s Next
- Learn next: Collar and Step Bearings - thrust bearings for water wheels and vertical shafts
- Combine with: Wear Assessment and Replacement - judging when the bronze washer has worn out