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Collar and Step Bearings

5 min read

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Thrust Bearings
Handling axial loads in vertical shafts and screws
Current
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Collar and Step Bearings
For water wheels, windmills, and vertical shafts

A collar bearing takes axial thrust on one or more rings turned on the shaft, bearing on a fixed bronze or babbitt face, and a step bearing carries a vertical shaft’s whole weight on a hardened pivot resting in a cup or bronze pad. Size either from the load divided by the face area, then check the heat; the allowed pressure depends on the face metal and on keeping oil at the contact.

Part of Bearings and Bushings

Two thrust arrangements for heavy axial loads: collars on a horizontal shaft, and the footstep or pivot under a vertical shaft in water wheels, windmills and mills.

The Face Area Sets The Pressure And The Speed Sets The Heat

A thrust bearing is limited by two numbers, as every bearing in this tree is. The pressure on the face is the axial load divided by the contact area. The heat is friction torque times angular speed, so it grows with how fast the shaft turns. A vertical water wheel turns slowly, 8-12 rpm per Gear Making, and can carry a heavy load on a small pivot; a fast shaft cannot.

A collar is a ring turned on the shaft that butts against a fixed bearing face; several collars share the load. A step bearing (footstep) is the foot of a vertical shaft, running on a hardened steel pivot in a steel cup or on a bronze pad with an oil groove, lubricated with heavy grease or an oil bath. Both are described in the carrier article.

What You Need

  • For a collar: a shoulder turned on the shaft or a keyed collar ring, and a fixed bronze- or babbitt-faced housing
  • For a step: a hardened shaft end or a hardened ball pressed in, a hard bronze or cast-iron pad, and a housing adjustable by threads or shims (Thrust Bearings)
  • Oil or heavy grease and a way to keep it at the contact
  • A way to measure axial play with a dial indicator or by feel

Method: Size And Build

Step 1: Find the thrust. For a mill, the runner stone alone can weigh 500-800 kg (Bearing Replacement), plus any other downward force.

Step 2: Choose an allowed face pressure. Thrust Bearings gives about 400-500 psi (2.76-3.45 MPa) for a babbitt collar face. That is a figure for babbitt only; no bronze figure is given, so for bronze copy a proven design or run under reduced load first.

Step 3: Compute the area, then the collar. Collar outside diameter squared = 4 x area / pi + shaft diameter squared.

Step 4: Cut the oil groove radially across the face, from the centre outward.

Step 5: Set the clearance. Thrust Bearings gives 0.003-0.005 inch (0.08-0.13 mm) axial clearance on each side of a collar. Too tight and it runs metal on metal; too loose and the shaft bounces.

Step 6: Fix only one end of a shaft that has thrust bearings at both ends; the other must float, or thermal growth will set up enormous stresses.

Worked Example

A collar on a 50 mm shaft takes 4,000 N, a load chosen for the example.

  • At 2.76 MPa the area needed is 4,000 / 2,760,000 = 0.00145 square metres = 1,449 square millimetres
  • Outside diameter squared = 4 x 1,449 / 3.1416 + 50 x 50 = 1,845 + 2,500 = 4,345, so the diameter is 65.9 mm
  • At 3.45 MPa the area is 1,159 square millimetres and the diameter is 63.1 mm

So a collar of 63-66 mm, rounded up, serves. Now the step case: a 75 mm spindle end (assumed) under a 800 kg runner. Weight = 800 x 9.81 = 7,848 N. Area = 3.1416 x 75 x 75 / 4 = 4,418 square millimetres, so pressure = 7,848 / 4,418 = 1.78 MPa. Taking the friction radius as one third of the diameter, 25 mm, and a lubricated coefficient of 0.02-0.06, the torque is 0.02 x 7,848 x 0.025 = 3.9 N m up to 11.8 N m. At 100 rpm (10.47 rad/s) the heat is 41-123 W. On a water wheel at 10 rpm the same footstep makes a tenth of that, 4.1-12.3 W. Dry, with a coefficient of 0.10-0.20, the heat at 100 rpm is 205-411 W (0.10 x 7,848 x 0.025 x 10.47 up to 0.20 x 7,848 x 0.025 x 10.47), and the step seizes.

Common Mistakes

  • Using the babbitt pressure limit for a bronze pad. It was given for another metal.
  • Fixing both ends of a shaft axially. Heating then jams it.
  • Neglecting oil at the centre of a step bearing. The pivot sits in a cup that needs oil fed to the middle of the contact.
  • Not checking axial play on a schedule. More than 0.010 inch (0.25 mm) of play in most applications warrants inspection.

Safety

A vertical shaft's weight is on the pivot

When the step wears or fails the stone, wheel or shaft drops. Jack and block the load before opening a step bearing, stop the drive so nothing can turn, and name the stop and its coast-down time. Never put hands or tools between a stone and its support.

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