Trailer Axle Anatomy — Complete Parts Guide
Trailer Axle Anatomy — Complete Parts Guide
Every part of a 3,500 lb EZ-Lube trailer axle, explained in plain English. Click any numbered hotspot on the diagram to jump to that part's description. When you know what you need, shop the collection directly.
1 Leaf Spring
The flat curved steel strap that the axle rides on. Spring packs absorb road shocks before they reach the trailer frame and cargo. Most 3.5K axles use double-eye leaf springs (a loop on each end) or slipper springs (one closed eye, one open slider).
2 U-Bolt Nuts
The 4 threaded nuts that clamp the U-bolts to the tie plate, locking the axle onto the leaf spring. Typically flanged locknuts (with nylon insert or serrated flange) so they don't vibrate loose.
3 Tie Plate
The flat steel plate that sits on top of the leaf spring and distributes the clamping force of the U-bolts evenly across the spring pack. Typically has 4 holes that align with the U-bolt threads.
4 Tube (Axle Tube)
The main structural beam of the axle. Hollow round steel tube (typically 2-3/8" or 3" diameter for 3.5K axles) with the spindles welded into each end. The tube carries the full weight of the trailer and cargo.
5 Spring Seat
The saddle welded to the axle tube that the leaf spring sits in. Positioned at the correct location along the tube for your axle's hub face dimension. On most 3.5K axles, the spring seat is pre-welded — not a separate part you install.
6 Spindle
The machined stub on each end of the axle tube that the hub rotates on. This is where the bearings ride. For EZ-Lube axles, the spindle has a drilled grease passage that lets you pump fresh grease through the spindle to the bearings without removing the hub.
7 Tube Assembly Wire
The brake wiring that runs through or alongside the axle tube, connecting the electric brake magnets at each wheel to the main wiring harness at the center of the axle. Usually 12-gauge stranded wire.
8 Wire Connector
The junction where the axle brake wires meet the trailer's main wiring harness. Usually a waterproof butt connector or weatherpack-style plug hanging off the center of the axle.
9 U-Bolts
The U-shaped threaded rods that wrap around the axle tube and up through the tie plate, clamping the axle to the leaf spring. Length and width must match your axle tube diameter (3" round for typical 3.5K axles).
10 Brake Assembly
The complete backing plate with brake shoes, magnet (for electric) or wheel cylinder (for hydraulic), springs, and adjuster. Mounts to the flange at the end of the axle tube with 4 or 5 bolts. For 3.5K axles, typical brake size is 10" × 2-1/4". Comes in left-hand and right-hand versions — they are NOT interchangeable.
11 Brake Nuts (Brake Mounting Bolts/Nuts)
The 4 or 5 bolts and nuts that fasten the brake backing plate to the flange on the end of the axle tube. Usually 1/2" bolts with lock washers or nylon-insert locknuts.
12 Inner Bearing
The larger of the two tapered roller bearings inside the hub. Rides on the inner race pressed into the hub, sits against the inner bearing cone on the spindle. For 3.5K axles, this is typically an L68149 cone with L68111 race.
13 Grease Seal
The rubber-and-metal disc pressed into the back of the hub, sealing against the spindle to keep grease IN the hub and water/dirt OUT. For 3.5K axles, typical seal is 1.719" × 2.565" (10-36 or 10-19 depending on spindle).
14 Hub Assembly (Hub & Drum)
The rotating part that the wheel bolts to. Contains the bearing races, holds the grease, and has the brake drum surface (combined hub-drum) where the brake shoes press. For 3.5K axles with 5-on-4.5" bolt pattern, hubs are typically cast iron with 1/2"-20 wheel studs.
15 Spindle Washer
The thick flat washer (sometimes with a keyed tab) that sits between the outer bearing and the castle nut. Distributes the nut pressure evenly against the bearing cone during preload adjustment.
16 Castle Nut (Spindle Nut)
The crown-shaped nut that threads onto the spindle end and sets the bearing preload. The "castles" (slots) align with the cotter pin hole through the spindle, locking the nut from backing off. For 3.5K axles, typically 1"-14 thread.
17 Cotter Pin
The bent wire pin that passes through a hole in the spindle tip and through the castle nut slots, locking the nut in position. ALWAYS use a new cotter pin — never reuse.
18 Dust Cap (Grease Cap)
The small metal cap that presses into the outer end of the hub, covering the castle nut and cotter pin, keeping dirt out of the bearings. On EZ-Lube axles, the dust cap has a rubber plug in the center that can be pulled out to access the grease zerk in the spindle.
19 Outer Bearing
The smaller of the two tapered roller bearings, sitting at the outer end of the hub. Rides on its own race pressed into the hub outer side. For 3.5K axles, typically an L44649 cone with L44610 race.
📏 How to Measure Your Trailer Axle
Before ordering a replacement axle or parts, you need three measurements. Write them down, bring them with you, or have them ready when you call.
1. Hub Face (HF)
Outside-to-outside distance between the two wheel hub mounting surfaces. This is the most important axle dimension — it determines track width and whether the axle fits your trailer frame. Measure from the outside face of one hub to the outside face of the other. Typical 3.5K axles: 60" to 95" hub face.
2. Spring Center (SC)
Center-to-center distance between the two leaf springs (or where the spring seats are on the axle). This is usually ~10" less than your hub face on standard axles. If you're replacing just the axle beam, SC must match your existing trailer's hanger spacing.
3. Tube Diameter & Spindle Type
3.5K axles typically use 2-3/8" or 3" round tube. Spindle type is either EZ-Lube (has grease zerk in end) or standard (no zerk, requires hub removal to grease bearings). You can see which by looking at the end of the spindle — if there's a small hole for a grease fitting, it's EZ-Lube.
Bring old parts to the counter. Our Howell location will match your existing axle to the correct replacement if you're unsure. Call ahead: 517-225-1991.
⚙️ EZ-Lube vs. Oil Bath vs. Nev-R-Lube
Three systems keep trailer bearings greased. Know which you have before ordering parts.
| System | How it works | Typical axles | Service interval |
|---|---|---|---|
| EZ-Lube | Grease zerk in spindle; pump fresh grease through spindle to bearings | Most 3.5K–7K (Dexter, Axis, AXL) | Every 6 months or 6,000 miles |
| Oil Bath | Hub is partially filled with gear oil; oil splashes onto bearings as hub spins | 10K and larger (Dexter, Lippert) | Check oil level quarterly, change annually |
| Nev-R-Lube | Sealed cartridge bearing; no user service | Dexter Torflex, newer RV axles | Replace cartridge at failure; no adjustment |
This page focuses on the EZ-Lube system shown in the diagram — the most common setup on utility, boat, and small cargo trailers.
🔢 Bearing Number Cross-Reference
Every trailer bearing has a number stamped on it (looks like "L44649"). Match your axle capacity to the correct bearing set — mixing sizes destroys them.
| Axle Capacity | Inner Bearing | Inner Race | Outer Bearing | Outer Race | Grease Seal ID |
|---|---|---|---|---|---|
| 2,000 lb | L44643 | L44610 | L44649 | L44610 | 1.249" (10-19) |
| 3,500 lb | L68149 | L68111 | L44649 | L44610 | 1.719" (10-36) |
| 5,200 lb | 25580 | 25520 | 14125A | 14276 | 2.125" (10-60) |
| 6,000 lb | 25580 | 25520 | 15123 | 15245 | 2.125" (10-60) |
| 7,000 lb | 25580 | 25520 | 14125A | 14276 | 2.250" (10-51) |
| 8,000 lb | 25580 | 25520 | 02475 | 02420 | 2.250" (10-51) |
| 10,000 lb | HM518445 | HM518410 | 15123 | 15245 | 3.376" (10-10) |
Always replace the bearing cone AND the race together. Never mix old race with new cone, or vice-versa — they wear together and no longer match after use.
❓ Trailer Axle FAQ
How often should I repack trailer bearings?
Every 12 months or 12,000 miles, whichever comes first. For EZ-Lube systems, you can add fresh grease through the zerk every 6 months as preventive maintenance. Any trailer that regularly gets submerged (boat trailers) should have bearings repacked annually regardless of mileage — water gets past the seal when a hot hub is plunged into cool water.
My hub gets hot after driving. Normal?
A warm hub is normal. A hot hub — one you can't comfortably touch — means trouble. Causes in order of likelihood: (1) bearings over-torqued (no room for grease film), (2) brake dragging (stuck shoe or adjuster), (3) bearing damage (too-loose preload, contamination, or heat-glazed rollers). Stop driving until diagnosed. Continuing on a hot hub will cost you a bearing at best, a spindle at worst.
Can I replace just one leaf spring?
Never replace just one. A new spring is taller than an old matched spring, which causes the trailer to lean to the old side. Always replace springs in pairs (both sides of the same axle) — ideally all four at once on a tandem axle.
What's the difference between a left-hand and right-hand brake assembly?
The self-adjuster mechanism inside the brake is handed — it only works correctly when installed on the correct side. The adjuster backs off when the brake is applied in reverse, so the ratchet direction matters. Using an LH brake on the right side means the brake never self-adjusts. Always order the matching pair for your axle.
How tight should I torque the castle nut?
Tighten the castle nut firmly while rotating the hub (50 ft-lb for 3.5K axles) to seat the bearings, then BACK IT OFF completely. Then re-tighten finger-tight while still rotating the hub, back off 1/6 turn (one castle slot), insert cotter pin. The hub should spin freely with no axial play. Over-torque is the #1 cause of premature bearing failure.
Do I need to match axle brand when replacing?
For the axle tube/beam itself — yes, or verify the replacement uses the same spring seat position, hub face, and brake flange bolt pattern. For bearings, seals, brake assemblies, and hubs, you can mix-and-match across brands as long as the specs are identical (L68149 is L68149 regardless of who made it). Dexter and Axis 3.5K hubs are typically interchangeable.
Can I convert a non-braking axle to add brakes?
Only if your axle has a brake flange (a 4-bolt or 5-bolt mounting ring at the end of the axle tube next to the spindle). Idler axles without brake flanges cannot be converted — you'd need to replace the whole axle with a braking version. Look at the end of your axle near the wheel: if you see 4 or 5 threaded holes in a ring pattern, you can add brakes.
What does EZ-Lube mean exactly?
An EZ-Lube axle has a drilled passage through the center of the spindle with a grease zerk at the very tip. When you pump a grease gun into the zerk, fresh grease travels through the spindle, comes out behind the inner bearing, and pushes old grease out past the outer bearing — all without removing the hub. You'll see the old grease ooze out the dust cap's rubber plug. This makes routine greasing a 30-second job instead of a 30-minute hub pull.
