Spherical Plain Bearing Applications & Advantages

Self-Lubricating Spherical Plain Bearings: material & selection guide

Embedded graphite or a PTFE liner

Both technologies remove the grease gun from the maintenance schedule, but they behave differently once the machine is running.

Tilt angle, load and size ranges

Two numbers decide whether a spherical plain bearing will fit the job: how far it can tilt, and what it can carry. Both are series dependent, so treat the figures below as planning values and confirm against the dimension table for the size you intend to use.

In practice the pv check is the one that gets skipped. Oscillating motion never builds a hydrodynamic film, so the contact runs in the boundary regime for its whole life; a bearing that looks generously sized against the static load can still run hot because the sliding distance per cycle is long and the reversing motion prevents heat from being carried away. Specify the cycle rate and oscillation angle, not just the load.

Spherical plain bearing or rolling bearing

Where the shaft rotates continuously at speed, a rolling bearing remains the normal answer and this page is not the right catalogue. The comparison is worth setting out:

• Size. For the same load, a spherical plain bearing is more compact in the radial direction than a comparable rolling bearing, which is why designers use them where space is tight.
• Misalignment. A spherical plain bearing tolerates angular error by design. A rolling bearing tolerates only minutes of arc before its life falls away.
• Shock. A sliding contact spreads shock over an area rather than through a line of rolling elements, so impact loading is handled without brinelling the raceway.
• Speed. This is where the sliding bearing loses. Friction is higher, heat is generated at the contact, and continuous rotation is normally limited to slow speeds.
• Starting torque. Sliding friction gives a higher breakaway torque than a rolling bearing, and stick-slip can appear at very low speeds with small oscillation angles. Where a control system needs repeatable positioning, a PTFE-lined type is usually the better of the two technologies.
• Life prediction. Rolling bearings have a standard rating life calculation. Sliding bearings are assessed from pv, wear rate and service history, so life is estimated rather than calculated to a standard.

Selection and installation notes

• Define the motion first. Oscillation angle, cycle rate and whether any continuous rotation occurs. Load alone is not enough to size these bearings.
• Size the pin. The inner ring bore runs on a pin or shaft finished to h6 or h7 with a surface finish better than about Ra 0.8 µm (32 µin). A rough pin wears the sliding layer quickly, whatever the layer is made of.
• Press on the right ring. Use a shouldered arbor that bears on the ring being fitted. Never press through the spherical contact — the load goes straight through the sliding surface.
• Do not split the outer ring more than necessary. Where the design uses a split outer ring, follow the assembly sequence so the halves meet squarely; a mismatched parting line produces a hard spot and uneven wear.
• Seal the joint. Embedded graphite tolerates contamination better than a PTFE liner, but both last longer behind a seal. In rock dust or slurry, a seal is the difference between a bearing that wears out and one that seizes.
• Allow for run-in. A short period at reduced load lets the transfer film establish before the bearing goes into full service.
• Check dimensions at 20 °C (68 °F). Measure bore, outside diameter and width on calibrated equipment; the method is set out in reading and measuring of bearing tolerance.

Typical installations include excavator boom roots and cylinder eyes, bulldozer ripper hinges and blade lift cylinders, hydraulic gate and dam gate linkages, hydropower turbine guide apparatus, marine deck machinery and mining equipment joints. The JDBS oilless spherical bearing is the catalogue series most often supplied for these duties.

When not to choose a self-lubricating spherical plain bearing

• Continuous rotation at meaningful speed. Sliding friction generates heat that cannot be carried away. Use a rolling bearing, or a lubricated plain bearing with an oil supply.
• Very small oscillation angles at high frequency. Below roughly 1° – 2° of movement per cycle, the contact does not refresh its transfer film properly and fretting wear appears. A PTFE-fabric type or a different joint design is usually better.
• Temperature above the liner limit. A PTFE-lined bearing at sustained temperatures above about 150 °C (300 °F) will not last; the bronze-graphite construction is the correct substitution.
• Applications needing low, repeatable starting torque. Sliding bearings have higher breakaway torque, and the figure changes with wear over life.
• Corrosive media with a bronze or steel ring. Seawater immersion or aggressive chemicals point to stainless rings with a suitable liner, plus attention to the galvanic couple.
• Where a sealed-for-life rolling bearing already meets the life target at lower cost. Self-lubricating joint bearings earn their place on load, misalignment and access; on a clean, slow, lightly loaded pivot, they may simply cost more.