What Is a 3502120-AW01 Wheel Hub Assembly? A Buyer’s Guide
3502120-AW01 is a manufacturer part number for one wheel hub assembly variant in the HJDP range from Jinan Huayuan Auto Bearing Co., Ltd., published at jnhjdp.com. The number is a catalogue reference, not a bearing size code: it does not state bore, load rating, flange pattern or vehicle fitment on its own. Resolving it takes three inputs — the drawing, a physical sample, and the mounting dimensions on the vehicle. This guide explains what such an assembly contains, how hub unit generations differ, how to read the drawing, and what to inspect before you order or fit it.
What Does the Number 3502120-AW01 Identify?
3502120-AW01 identifies a single wheel hub assembly reference; by itself it says nothing reliable about dimensions, load rating or which vehicle it fits. That is the first thing to settle before a part is quoted or ordered. A hub assembly number is a manufacturer’s functional code, and the only dependable way to turn it into a purchasable specification is to match it against a drawing and a physical sample.
Why a hub number cannot be decoded like a bearing size
A bearing designation such as 6205-2RS is self-describing: series, bore and sealing are fixed by a published standard, so any catalogue returns the same envelope dimensions. Hub assembly numbering has no equivalent standard. It is built from a group reference that places the part in the wheel-end family, plus a suffix — AW01 here — that separates one application or design revision from another. Assemblies sharing the 3502120 group can still differ in flange diameter, stud circle, bearing preload, tone-ring type and axial offset. A shared prefix is therefore not evidence of interchangeability, and a catalogue cross-reference to an OE number is only a starting point: it is confirmed against the drawing and a sample, not against a table in a catalogue.
Where a rear hub assembly sits, and what it carries
A rear hub assembly does four jobs at once. It locates the wheel on the axle stub or trailing-arm spindle, carries the brake disc or drum, provides the machined pilot and stud circle the wheel is clamped against, and — on ABS or ESP equipped vehicles — spins the encoder that the wheel-speed sensor reads. The loads it sees are a vertical wheel load, a lateral load in cornering, a torque reaction under braking, and on a driven axle a drive torque as well. Many C-segment cars run a non-driven rear axle and therefore a lighter double-row ball unit at the rear than at the front, but that is a drawing fact about a specific part, not a rule that can be applied across a range.
One group, several distinct assemblies
The HJDP catalogue shows how much a suffix can change. The same 3502120 group contains a rear wheel hub assembly reference built to its own drawing and a second rear hub reference with a different suffix, and the two are not interchangeable simply because the group matches. Ordering from a remembered vehicle name, or from a prefix alone, is how wrong parts reach a shelf and come back as returns. Practical rule: the number selects the drawing, the drawing defines the part, and the sample confirms the drawing.
What Is Inside a Wheel Hub Unit, and How Do the Three Generations Differ?
A wheel hub unit is a sealed, pre-greased double-row angular contact bearing packaged with one or two mounting flanges, and the generation number describes how much of the wheel end is built into that package. Generations are not quality grades. They describe the division of labour between bearing, knuckle and hub, and that division decides what you buy, what has to be pressed, and who is responsible for preload.
First generation: the bearing on its own
A first-generation unit is a double-row bearing with two inner rings and one outer ring, pressed into the knuckle or the hub and retained by a circlip or by the axle nut. Preload is set during installation by the torque applied to the axle nut, which makes it the cheapest part to buy and the most sensitive to fitting. An impact wrench, a reused staked nut or a bearing pressed in by its inner ring can shorten its life to a few thousand kilometres, and both seal faces are exposed to dirt at the moment of pressing.
Second generation: one flange built in
A second-generation unit integrates one flange with the outer ring or the inner rings, so the part is bolted or pressed to the knuckle as a single sealed assembly. Preload is set and locked by the bearing maker, which means installation only has to seat the unit squarely and torque it correctly. The ABS encoder ring is normally carried on the sealed face, which is also the face most easily damaged by a careless press or a dropped part.
Third generation: the complete wheel end
A third-generation unit combines hub flange, bearing rows and mounting flange in one bolt-on assembly; nothing is pressed in the field and the whole wheel end is replaced as a single item. Installation error drops sharply, but the trade-off is that a failed element condemns the entire assembly, and the encoder becomes an integral, fragile component instead of a separate ring that can be inspected cheaply.
| Feature | Generation 1 | Generation 2 | Generation 3 |
|---|---|---|---|
| What the box contains | Bearing with two inner rings | Bearing with one integrated flange | Complete hub with both flanges and encoder |
| How it mounts | Pressed into knuckle or hub, circlip or axle nut | Bolted or pressed on the flanged side | Bolted directly to the knuckle |
| Who sets preload | Installer, through axle nut torque | Bearing manufacturer | Bearing manufacturer |
| ABS interface | Separate tone ring, or none | Encoder ring on the bearing face | Magnetic encoder integrated in the seal |
| Tooling needed | Press, driver set, torque wrench | Press or bolts, torque wrench | Socket set, torque wrench |
| Typical weak point | Fitting damage, seal life | Knuckle bore fit, seal lip | Encoder damage, bolt torque |
Which Bearing Type Suits Which Load Path?
Ball-based units cover most passenger-car wheel ends; tapered roller bearings take the combined radial and thrust loads of commercial vehicle hubs, differentials and trailer axles; cylindrical roller bearings are chosen where radial capacity and stiffness matter and thrust is handled elsewhere. The load case sets the choice, not the price list, and a substitution across types changes the stiffness of the whole wheel end.
| Bearing type | Radial capacity | Thrust capacity | Stiffness | Typical position |
|---|---|---|---|---|
| Double-row angular contact ball | High for its size | Moderate, both directions | Moderate | Passenger car front and rear hubs, second and third generation units |
| Tapered roller, single or paired | High | High, one direction per row | High | Light truck and trailer wheel ends, differential pinion and carrier, some SUV rear axles |
| Cylindrical roller | Very high | Low | Very high | Gearbox shafts and pinion supports where a separate bearing takes thrust |
Tapered roller bearings in wheel and driveline service
Tapered bearings use separable cones and cups with opposed rows, and the running clearance is set at assembly by shim, spacer or adjusting nut to a stated end play or preload. That adjustability is their strength in truck and trailer hubs and their weakness in inexperienced hands: set too tight they fail by heat discolouration and grease breakdown, set too loose they fail by impact loading on the roller ends. They tolerate heavy combined loads and shock, which is why they dominate commercial wheel ends and differential pinion heads.
Cylindrical roller bearings where thrust is carried separately
Cylindrical roller bearings use line contact between roller and raceway, giving very high radial capacity and stiffness with almost no axial capability. They appear in gearbox mainshafts and pinion supports where a separate thrust bearing or a tapered pair handles axial load. They are not a drop-in substitute for a ball hub unit or a tapered set, because the surrounding geometry assumes the thrust path is closed elsewhere.
The parts around the hub
Wheel-end work rarely stops at the hub. Tensioner and idler bearings are sealed deep-groove or double-row units loaded by belt tension rather than vehicle weight, so their failure signature is noise and belt wander rather than wheel play. Clutch release bearings are angular contact thrust units that run under constant preload from the release mechanism. Drive torque passes through constant velocity joints: a right-hand CV axle assembly in the HJDP range shows the arrangement, with an outboard joint that carries steering angle and an inboard joint that plunges for suspension travel, and the matching left-hand assembly follows the same logic on the opposite side. Handedness is a drawing attribute, not something to infer from the part number pattern.
How Do You Read the Drawing Before Approving a Sample?
Read the drawing from the interfaces inward: mounting face and bolt pattern first, wheel interface second, ABS interface third, and only then the bearing internals. Most wrong-part problems are interface problems, and interfaces are the cheapest characteristics to verify before money is committed.
- Mounting interface. Flange bolt count, thread size, pitch circle diameter, and the offset from mounting face to wheel flange face. A two-millimetre offset error either preloads the bearing or pushes the disc out of the caliper bracket.
- Wheel interface. Pilot bore diameter, stud thread and knurl, stud length, and the position of the brake disc retaining screw hole.
- ABS interface. Encoder type — magnetised seal or toothed tone ring — pole count, sensor mounting position and running air gap.
- Bearing internals. Ball or roller, number of rows, contact angle, bore, outer diameter and width.
- Tolerance and clearance callout. The dimensional standard and class, plus the clearance or preload group, stated numerically rather than by habit.
- Sealing and lubrication. Seal design on each side, grease type and fill quantity, and whether either side is directional.
- Markings. Part number, batch code and any orientation arrow; a directional seal fitted backwards will pass a bench spin and leak in service.
- Sample confirmation. Compare the drawing against a physical unit — offset, pilot bore, encoder and stud pattern — and record the approved revision before volume production.
Clearance, preload and tolerance class are three different things
Tolerance class controls geometry: bore and outer diameter deviation, runout, flange concentricity. Hub units are commonly called to a P6-equivalent class or better, but the drawing governs and the class should be stated on it. Clearance controls internal play between rolling element and raceway: a C3 group is not an upgrade over CN, it suits a different thermal expansion case. Preload is negative clearance. On a sealed second or third generation unit it is set at manufacture and cannot be adjusted, so a rough-running new unit is a reject, not something to be tightened; on a first-generation bearing or a tapered set it is set at installation and verified as starting torque or end play.
How Do Seals, Grease and Heat Treatment Decide Service Life?
Seal design, grease choice and heat treatment decide service life more than the steel grade alone, because a correctly hardened bearing still fails early once contaminated grease or a damaged lip lets water and grit in. These three items are also where low-cost copies diverge most from a specified part, and where a drawing review pays for itself.
Seal and encoder designs
Common arrangements are a single lip with a garter spring, a double lip with an internal grease retainer, and a cassette or labyrinth seal with a steel slinger that keeps water away from the lip. On ABS units the encoder is often moulded into the seal, so the seal is simultaneously a signal generator: a cracked or contaminated encoder produces an ABS fault with no mechanical symptom at all. Seal orientation and press depth are specified dimensions, not fitting preferences.
Grease type and fill
Hub units are normally greased for life with a lithium complex or polyurea thickener chosen for the operating temperature and the seal elastomer. Fill quantity is a drawing value: too little starves the outer row on the loaded side, too much causes churning, heat and seal weep. Thickeners of different chemistry should not be mixed when re-greasing a serviceable bearing, because the resulting consistency can soften and escape past the lip.
Material and heat treatment
Rings and rolling elements are typically made from through-hardening bearing steel such as 100Cr6 or GCr15, hardened and tempered to roughly 58 to 62 HRC and dimensionally stabilised for the service temperature. Flanges are usually medium-carbon or micro-alloyed steel, with induction hardening at the stud seats and bearing seat; the hardness pattern there decides whether stud holes elongate and whether the flange face frets against the knuckle. Cages are glass-filled polyamide in many passenger units and steel where temperatures run higher. Each of these is a line on the drawing that can be checked against a sample.
How Should a Hub Unit Be Installed and Replaced?
Hub units fail early for installation reasons far more often than for manufacturing reasons, and the three recurring causes are pressing on the wrong ring, dirt entering during fitting, and incorrect nut torque. The sequence below applies to a typical bolt-on or press-in rear hub and should be adapted to the specific drawing and to the vehicle maker’s torque values.
- Clean the knuckle bore, the spindle and the mounting face, and remove corrosion and old thread-locking residue; measure the bore if the old unit spun or showed play.
- Support the brake caliper so the hose carries no weight, and hang it clear of the hub face.
- Remove the old unit with a puller or press, never with a hammer on the flange or the spindle.
- Press or draw the new unit in by its outer ring or mounting flange, checking squareness continuously; force through the inner ring preloads the rows and damages the raceways before the car moves.
- Fit a new axle nut where the original is a staked or self-locking type; a reused nut loses clamping load and lets preload drift.
- Torque to the vehicle maker’s figure with the wheel fitted and the car on the ground where the design calls for it, using a calibrated wrench rather than an impact gun.
- Reconnect the wheel-speed sensor, check the encoder face for debris, and confirm the connector latches fully.
- Road test, then re-check for play and for ABS warning lights before the vehicle is released.
What Do the Common Failure Modes Tell You?
Noise that rises with road speed points to the bearing rows; play at the wheel points to preload loss or a worn knuckle bore; an ABS light after replacement usually points to the encoder or the connector rather than the bearing. Reading the symptom before dismantling saves a second part and a second labour charge.
| Symptom | Most likely cause | How to confirm |
|---|---|---|
| Hum or growl rising with speed, tone changing in bends | Raceway or ball spalling, brinelling from impact | Road test, then spin the hub with the wheel off and feel for roughness through the flange |
| Visible play at the wheel | Worn bearing rows, lost preload, worn knuckle bore | Check play with the wheel raised; if play persists after correct torque, measure the bore |
| ABS warning after fitting | Encoder damaged, wrong encoder type, air gap wrong | Read fault codes, inspect the encoder for cracks and metallic debris |
| Grease weeping at the seal | Lip damaged during pressing, or overheating | Check for brake drag and for a scored seal journal on the spindle |
| Blue or discoloured flange | Brake drag or excessive axle nut torque | Compare brake temperatures side to side and verify torque value used |
| Clicking on full lock | Constant velocity joint rather than hub | Inspect the boot for splitting and the joint for play through the shaft |
Frequently Asked Questions
Is 3502120-AW01 the same part on both sides of the vehicle?
Not necessarily. Some hub references serve both sides, while others are handed because of the ABS sensor position, the brake caliper mounting or the park brake arrangement. Whether a specific number covers left and right has to be confirmed against the drawing and a sample rather than inferred from the number itself, and the same caution applies before quoting a pair to a workshop.
Can a first-generation bearing be replaced with a third-generation hub unit?
No. The generations describe different knuckle interfaces. A third-generation unit bolts to a machined mounting face with its own bolt pattern, while a first-generation knuckle has a bored seat for a pressed bearing and relies on the axle nut for preload. Converting between them means changing the knuckle, which is a vehicle engineering decision, not a parts substitution.
What usually causes a hub to fail within the first few thousand kilometres?
Installation. Pressing on the inner ring instead of the outer, letting grit into an open bearing, reusing a staked axle nut, torquing with an impact wrench, or fitting a directional seal the wrong way round all shorten life sharply. When a recently fitted unit fails, confirm it was seated squarely and that the nut reached the specified torque before blaming the part.
How do I confirm fitment before placing an order?
Send the part number, the vehicle details and photographs of the old unit: mounting face, wheel pilot, ABS connector and any markings on the casting. Compare the drawing dimensions — offset, bolt circle, pilot bore, encoder type — and approve a sample before volume. A catalogue cross-reference alone is not confirmation; the drawing and a sample are.
