Drum & Bearing Inspection
- Determine when to re-machine or replace brake drums based on wear and out-of-round measurements.
- Perform proper bearing and cup replacement procedures to ensure mechanical compatibility.
- Manage grease types and armature surfaces during hub and bearing maintenance.
- Brake drum micrometer — Measure drum diameter to verify if wear exceeds .030" or if out-of-round exceeds .015".
- Bearing race driver — Install new cups into the hub, ensuring the cup and cone are replaced as a matched set.
- Degreasing solvent — Remove all old grease when changing types to prevent mixing incompatible bases.
- Drum lathe — Reface the armature surface or machine the drum within the max diameter limits of 10.09" or 12.09".
Bearings are always replaced as a set — one cone and one cup.
Magnets should be replaced whenever the armature surface is refaced, and vice versa.
Never mix lithium, calcium, sodium or barium complex greases. Remove all old grease when changing types.
How It Works
Bearing Set Integrity
Bearings are precision-matched components. They must always be replaced as a set consisting of one cone and one cup. This ensures proper fitment and load distribution within the hub assembly.
Grease Compatibility
Lubricants with different thickener bases do not mix. Technicians must never mix lithium, calcium, sodium, or barium complex greases. When changing grease types, all old grease must be removed completely.
Armature and Magnet Maintenance
The relationship between the magnet and the armature surface is critical for braking performance. Magnets must be replaced whenever the armature surface is refaced, and the armature surface must be refaced if a new magnet is installed.
Drum limits
| Re-machine drum if wear exceeds | .030" |
| Re-machine drum if out of round by more than | .015" |
| Replace drum if scoring/wear exceeds | .090" |
| Max machined diameter — 10" / 12" drum | 10.09" / 12.09" |
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| Drum shows visible scoring or significant material loss. | Measure depth of scoring and overall wall wear. | .090" | Excessive wear or scoring beyond this limit requires drum replacement. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| Brake pulsation or uneven braking performance. | Measure drum for out-of-round condition using a micrometer. | .015" | The drum is out of round beyond the machining limit and requires resurfacing. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| Drum surface shows moderate wear but remains under the replacement limit. | Measure current wear against the re-machining threshold. | .030" | Wear exceeding this value necessitates re-machining to restore the braking surface. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| Uncertainty if a drum can be safely machined further. | Measure the internal diameter of the 10" or 12" drum. | 10.09" / 12.09" | The drum has reached the maximum machined diameter limit and must be replaced. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
At the Bay: Diagnose the Case
CASE 1 / 4
Drum shows visible scoring or significant material loss.
- Test
- Compare
- Correct
Select the next check
Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014). A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.
Common Bay Mistakes
- ✕ Replacing a bearing cone while leaving the old cup in the hub.
- ✕ Mixing lithium-based grease with barium or calcium complex greases.
- ✕ Refacing the armature surface without installing a new magnet.
- ✕ Machining a 12" drum to a diameter greater than 12.09".
- ✕ Failing to remove all old grease when switching to a different grease complex.
Quick Check
Practice only — not scored.
Hands-on: Shop Test Equipment
Dial Indicator — Bearing End Play
Magnetic-base dial indicator (0.001 in) on the hub face, spindle nut adjustment
🛠 Equip your bay for this test →Symptom: Bearing service — set end play after repack.
Spec limits used
- 0.001–0.010 in — Allowable end play, double-nut wheel end (Dexter 9-15K service manual, p.64)
- 0.008 in — Nev-R-Lube 50 mm max axial end play (Dexter 059-831-00)
- illustrative only — Face runout limit (General shop practice)
Simulated readings for practice. Always follow the manufacturer's procedure and values on a real trailer.
Source: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) — 5