Travels with an Oka

Free Wheeling Hub Maintenance

2012-02-11T19:31:00.002+10:30

Many Oka owners have AVM freewheeling hubs and several (including me) have had on-going problems with them.

Several times a hub has broken such that they either won't engage or disengage 4wd, (they just click as the wheel rotates), twice the fixing screws have come loose and once the outer hub has fallen off and disappeared completely leaving us without a 4wd capability.

So I've analysed how they work and what can go wrong, and considered what can be done about it to ensure you have 4wd when needed on an outback trip and 2wd when you don't. Some of this information may also be relevant to Warn and other makes of hubs (eg Superwinch) which probably work on a similar principle.

The AVM Model 439 Free Wheeling Hub

If you have SuperWinch Free Wheeling Hubs like this, you will probably also have the same problems. They look like rebadged AVM's and I know of one which is stamped with the same casting number, 439. The SuperWinch part number is 400439.

AVM Hub Components

The AVM hubs comprise 2 main assemblies, an outer bolt-on assembly which contains the Free/Lock controls and a sliding locking ring, and an inner gear which engages the internal hub splines and drives the hub round and also holds the splined drive shaft roughly central.

Outer Assembly

The outer assembly comprises an aluminium outer frame, a plastic control lever, a grooved plastic cylinder, a large spring and a steel locking ring. It bols on to the inner assembly via 6, 2 inch long countersunk socket head screws.

The locking ring and spring behind the outer assembly shown in the Lock position

The locking ring is attached via a spiral spring to a plastic ring with 3 tangs on it to engage in the grooved cylinder and it's alignment is maintained by 2 thin roll pins inserted into the aluminium frame.

Inner Assembly

The inner assembly comprise a solid steel ring casting with external splines held in by a large internal circlip inside the hub, plus a central spline gear which is free to rotate but is held into the larger ring by a rear circlip. It is also held in by a circlip around the front of the central splined drive shaft.

The rectangular cams and gear teeth on the locking ring on the outer assembly on the left (shown in the retracted Free position) engage with the inner gear assembly on the right to lock the hub

Warn use similar locking ring and fixing screw principles

Hub Operation

When the lever is moved to the Lock position, the cylinder is rotated by the control lever and the tangs on the locking ring slide down the grooves. The spring expands pushing the locking ringing into the inner assembly, locking the splined central shaft to the outer hub by engaging the gear teeth and the ring of 6 rectangular cams.

The natural or fault position of the control is the Locked position and turning the lever to the Free position actually pulls the locking ring away from the inner gear. Indents on the grooved cylinder hold it locked or unlocked.

No maintenance is normally required, but if they are removed, cleaning the plastic surfaces and lightly oiling them will maintain smooth operation. Also oil the central splined gear where it rotates continuously inside the back of inner assembly.

Failure Modes

The inner mechanical components of the AVM hubs are solid and strong I have never had a problem or concern with the mechanics of its operation. If anything there ever broke it would indicate something far more seriously wrong with the stub axle/spindle or bearings than the free wheeling hub. However anything can break and I have seen the equivalent locking ring broken on Superwinch and Warn hubs.

The outer assembly is held in by 6 long screws into the inner assembly and these screws can come loose, since they have no locking mechanism, potentially causing the outer assembly to fall off.

Without outer assemblies fitted, the vehicle can only operate in 2 wheel drive as there is no locking ring to couple the splined shaft to the hub. [OK, with a diff lock, 3wd is theoretically possible].

The inner gear components are held in by 3 separate circlips so it's very unlikely they can come out in operation. They contain no plastic or weak components so it's a very strong mechanism.

The plastic control components in the outer assembly however are weak and poorly designed and it is these that are most likely to cause a failure.

The plastic parts that can break. The ring with tangs, the grooved cylinder and the control lever

In my experience the primary problem is the tangs on the plastic ring which either wear away or break off. This leaves the hub either unable to engage or disengage 4WD, or, when one tang breaks off, the hub is left partly engaged and clicking as the wheel rotates. There is only a 90º turn in the control mechanism (unlike Warn's 325º) which puts a strain on the plastic tangs.

2 broken plastic rings. The top one is complete except for one broken tang. The lower one has all three tangs but the sides are shattered.

The grooved cylinder can also wear so the indents no longer keep the hub in the selected position and/or the clips on the inside of the control lever fail to hold the grooved cylinder so that control is lost. The hub may then become locked, even though the lever looks like it's in the Free position. This is a "fail-safe" situation (ie 3 or 4wd) but can catch you out if you are not aware of it. Where this is suspected, I turn the front drive shaft by hand until I know what's engaged or not.

I've addressed this problem with AVM in Brazil and asked if there are stronger "plastic" components or spare parts available and this was their response:

Dear Mr.Ribbans,

really the nylon pieces are the weakest of hub components and might last just 6/7 years in standard conditions of use . We do not sell internal components to avoid installation faults and safety problems. Therefore we commercialize just the service kit shown in the attached picture and the nylon cap dial. Anyway please contact our local distributor at broach@donkyatt.com.au and ask him if can import the complete nylon set ( 3 pieces) . In that case we will do an exception for you and supply it.

thank you

Export dep.

I've contacted Don Kyatt and they are looking into the availability of spares.

The "service" kit (AVM #4.439) comprises only 2 sets of replacement screws, large O-rings and circlips for the inner hub and splined shaft, which is pretty useless.

AVM 4.439 Service Kit

Faults on other makes of hubs

Although Warn is touted as a superior make, a brief search indicates there are cases of failures in their hubs too, but their problems seem to be mainly shattered locking rings rather than the control mechanism. The Warn locking ring appears to be slightly thinner than the AVM equivalent but has more cams.

What to do if a FW hub fails

In an outback situation, loss of 4wd is the most important consideration but can be easily prevented or rectified with a few precautions and/or some critical spares:

Firstly it is preferable/essential that the the outer hub doesn't fall off since it contains the all important locking ring, without which 4wd is not possible. (We were on a bitumen road to Melbourne when ours disappeared so it can happen at any time).

The outer hub is fixed by 6 screws which are 2 inch long possibly 6/32 UNC countersunk socket headed and deeply recessed into the aluminium frame. If they are over tightened or Loctited to avoid them loosening they can be very difficult to remove and I've had to drill several out. The heads are either small Allen sockets, which round off very easily, or Torx sockets which are easier to remove.

[Note: to remove a stubborn or rounded Allen head screw, you can hammer an equivalent sized Torx driver into the Allen socket to improve the grip, the sharp points cut their own shape. If you must reuse a damaged screw, cut a slot in the head first so a flat bladed screwdriver can be used for later removal].

Screws and the outer hub can be retained by several means (suggested by Oka Owners Group Members):

Loctite the long fixing screws. This will work but makes their removal potentially difficult and might mean drilling them out.

Put tape over the heads of the screws so they can't come out, or if they try too it's easier to spot.

Screws held captive by aluminium tape

Fill in the holes above the heads of the screws with silastic. This will work but it's also potentially difficult to remove.

Drill and re-tap the screw holes on the inner gear and aluminium frame to allow larger bolts and lock washers to be used with bigger heads that you can actually get a spanner on. Two larger bolts would probably suffice and M5 is probably the largest bolt size that could be fitted, M6 might exceed the space available and interfere with the internal circlip. These bolts could then be Loctited successfully since removing them is mechanically easier.

Bolt a strip of steel/aluminium across the front of the aluminium frame to prevent it from falling off, and secure the strip on to the hub steps, if fitted, or to the hub using a large hose clip or even a large tie-wrap. However this might impede operation of the control lever.

Fabricate a steel ring clamped to the hub (or use the step ring), with 2 or more claws welded around it which fit out and over the hub screws to retain them and the outer assembly. The claws will not impede access to the control lever and the ring can double as a non-slip step ring. A thin flat ring could be welded to the ends of the claws to retain all the screws and avoid them catching on passing flora.

Although I'm currently implementing the last suggestion, drilling and and tapping to fit larger hex or Allen headed bolts is probably the most effective but dramatic solution, and doesn't affect wheel removal.

Secondly, in the event that the outer assembly does fall off without trace, or if its internal components fail, there is still a quick and easy way to engage 4wd as long as you have a few critical components available:

Either a complete spare outer hub assembly (obviously) or,

Parts from a failed outer hub assembly:

the aluminium frame (the alignment roll pins are not necessary and could be removed),
a locking ring,
the large spring, and
a few of the long screws.

These items can be recovered from previously failed hubs and/or are not heavy items to carry as spares.

Components needed to engage 4wd

To lock the hub into 4wd, insert the locking ring, bevelled edge first, into the inner hub gear and bolt the aluminium frame on to the hub, compressing the large spring between them. Only a minimum of 2 or 3 screws are required to hold the outer frame on since there's not a huge force against it, and in 4wd, all the hub components rotate together. Put some tape over the screw heads so they can't come loose.

This can be done in a few minutes since no wheel removal or jacking is required, and the centre hole on the frame can be covered by sticky tape to keep dust out.

Just a locking ring alone would suffice if you could engineer a way of holding it into the hub, eg screwing a plate or strip across the hub securing it with the long fixing screws, but if you've got a locking ring, you've probably also got an aluminium frame.

To revert to 2wd, remove the locking ring and spring and replace the aluminium frame.

The outer frame and spring push and hold the locking ring into the inner gear assembly, locking the hub

What other precautions can be taken?

The inner assembly is held in place by a large circlip which sits in a lipped groove in the hub, behind which is a bevelled edge prior to the internal splines. It is therefore possible and desirable to fit 2 circlips.

The groove is wide enough for 2 circlips to hold in the inner assembly

The reason is as follows. The locking ring begins to engage the inner gear when the spacing from the outer hub face to the inner gear face is about 6mm. This very close to the normal unlocked separation of the inner gear and locking ring, which is why a single broken tang can cause clicking. The long screws actually draw these faces together when tightened and anything that can increase the spacing minimises this problem. Fitting 2 circlips will achieve this.

It has been also suggested that the length of the splined shaft gear or location of the circlip groove can affect operation of the hub, but in my case, I could not see any way that the gear or shaft could impact the plastic components. The drive shaft position is controlled by the thrust surface on the rear of the wheel hub and the ends of the splines remain about 10mm from the control components. Of course replacement or other makes of drive shaft may have different lengths and the circlip maybe in different locations along the shaft.

If the central spline gear seems to be too long, don't fit the circlip on the splined shaft, it can't go far and the gear is still held to the inner assembly by a rear circlip. Others have machined the gear shorter to allow fitting of the circlip but that depends on the length of the drive shaft which I assume might vary between makes.

Spare Parts

I have had around 3 or 4 failures of AVM hubs, all resulting from broken plastic parts, plus one lost outer assembly caused by screws working loose, and twice I've had to buy replacement sets. So I now have a stash of spare parts which can be re-assembled into useable spares.

My pile of AVM free wheeling hub spares, not including the 2 on the Oka

All the mechanical components are reusable, but very few of the plastic parts are

Disassembly or Repair of the Outer Hub Assembly

If you need or want to disassemble/reassemble the outer hub there are few things to be aware of.

The control lever is held to the grooved cylinder by 2 thin clips on the back of the control lever. When these are locked together, the locking ring and the spring are all held compressed.

To remove the control lever, use a round bar (I used a socket extension bar) held against the inside to the lever and give it a sharp tap. It should pop out and all the components can then be removed.

To reassemble the outer hub, note that he the control lever has 2 slots on the rear which engage the 2 tabs protruding from the grooved cylinder. These tabs are not the same width and must be oriented correctly so the Lock/Free positions line up. Refer to the photo of plastic items above.

Also the grooved cylinder can be oriented in 3 different positions on the tangs so ensure that it is correctly aligned so that when engaged with the operating lever, the Lock/Free positions are correct.

When the components are correctly aligned, holding them all compressed on to a solid surface, use a socket which will just fit inside the cylinder and tap it to engage the control lever clips. If one tap is not enough, check the alignment of the control lever with the grooved cylinder tabs.

Note, each time this is done, or if the parts are not aligned properly, there is a risk of the plastic edges breaking away, eventually causing them to fail, so don't do this too often. I have successfully done this a few times, to construct a working outer hub from components from several failed hubs but they are not designed to be disassembled.

Oil the plastic parts lightly to minimise wear and allow them to slide smoothly.

Differences in Hub Design

When rebuilding a spare inner hub, I observed that there are difference in the design of the outer ring.

Both types are marked 439, which is the AVM part number for a hub to suit a Dana 60 internal mounting and 30 spline shafts.

Note the opening (not the spine gear) in the rear of the inner ring on the left is larger than the one on the right.

The inside diameter of the left hand gear is 50mm whereas the right hand one is only 49mm. Although it doesn't sound much, the inside surface of the outer ring, which rotates continuously with the wheel, is what supports the splined gear, which is stationary in 2wd, and 1mm can make a difference. On the larger ring, the spline gear almost falls through the hole.

I suspect the larger ones are early models since all our later models have been the 49mm size.

Both rings are marked "439"

For reference, AVM also make a similar hub for 35 spline shafts, model 439K, which is the model recommended for the Oka NT and is listed in their catalog. The 30 spline model 439 is listed under Ford F350, Dana 60.

Starter Solenoid/Armature Failure Analysis

2011-10-29T18:20:00.001+10:30

Starter Motor Failure

Our starter motor failed in mysterious circumstances at Eucla, half way across the Nullarbor Plain. We'd stopped to use the facilities at the Eucla Roadhouse and afterwards, the starter motor refused to function.

The solenoid on the starter motor had been giving intermittent trouble for a couple of years and usually tapping the start relay connection was enough to get it going, but not this time. I replaced the solenoid but the motor was still faulty so we eventually assembled a working one out of spare parts and found that a replacement armature was also required to get it going.

Further analysis on the faulty components was obviously required after we arrived home. This is it.

Solenoid Failure

In Eucla I took the solenoid off and checked it electrically. Not surprisingly one of the windings had failed internally.

Since these are not repairable items (at least, not without a lot of effort and desperation) we ordered a new one which took a couple of days to be delivered from Perth. In the event that didn't fix the starter problem and we had to wait until we could bump start the Oka and travel on to Coolgardie, without switching the engine off, where I could fix or replace the motor.

The second, concurrent, problem was a faulty armature, which turned out to be a winding fault, rather than excess commutator wear (which would have been simple to fix). Why and how we could have 2 problems which occurred at the same time is still a mystery, unless the solenoid was in one of its in-between failure periods and it was actually the armature which failed first. I guess we'll never know but it did delay us for a few days, luckily right outside the Eucla Motel.

Opening up the Solenoid

The starter solenoid is assembled into a steel cylinder, the top edge of which is peened over during the manufacturing process to hold the contactor plate in place, so it can't be easily be disassembled.

Electrical operation of a starter solenoid is explained in this useful article.

I first unsoldered the winding wires from the terminal posts (there are 2 wires on one terminal) using a solder sucker and desoldering braid to remove the solder. The thinner wire came completely out indicating a broken or fused wire in the hold-in winding. Then I cut through the skin of the cylinder near the contactor using a hacksaw and removed the internal components.

Where I cut the solenoid cylinder

The contactor removed after cutting though the cylinder

The windings are wound around a plastic former on a steel base plate which is held in place by a large circlip.

The fused hold-in winding and circlip holding the coil base plate in

After removing the circlip and sliding out the coil assembly, it was immediately obvious that there had been a burning event and both windings were burnt black at the terminal end. Although the hold-in coil had failed, it was unclear whether the thicker pull-in winding had overheated, causing the thinner hold-in winding to fail, or vice versa.

Windings removed

The burnt windings.
The thinner hold-in winding is wound over the top of the thicker pull-in winding, separated by paper insulation.

Solenoid Repair Possibilities

The solenoid is a fairly simple electrical device and if necessary it could be rewound and reassembled by cutting out the burnt sections of wire (the insulating enamel may have been damaged by heat causing further short-circuits) and rewinding them. The windings both have around 100 turns on them so a few turns missing would not affect operation appreciably. The cut off cylinder could be repaired using a hose clip, tie wraps or similar to hold the contactor plate in place.

Pull-in winding after removal of the hold-in winding

Although the solenoid could be repaired if the situation was really desperate enough, it's probably a better choice to carry a spare at around $100 and save all the potential angst.

Contents of the solenoid
"To reassemble, simply reverse the disassembly process"

Causes of Failure

There was no obvious mechanical or electrical reason for the winding to fail, but possible causes are:

Prolonged engagement of the starter motor, allowing the windings to heat up to the point where the insulation breaks down, causing a partial short circuit and leading to eventual winding failure, or
Heat from the exhaust system which runs nearby, aggravating the internal self heating of the solenoid windings and leading to the same result. This is most likely to happen when restarting an already hot engine (which coincidentally is exactly what happened to us in Eucla), or
Shit just happens when you least expect it.

The solenoid on our Oka is very close to the exhaust

Using Iskra starters (and possibly other makes), there are 2 frame types with different locations for the starter solenoid:

on the side of the motor (which is close to the exhaust pipe on our Oka), or
below the motor where it's more exposed to impact damage (but no more so than the engine sump would be).

There is the merest possibility of the front drive shaft or its capture hoop touching the solenoid in the lower position at maximum axle articulation, but that seems most unlikely to happen in practice as the bump stops/airbags would have to completely fail to do their job.

Solenoid located on the side of the motor, like ours,
Iskra IM527

Solenoid in the lower, but more exposed location,
Iskra IM315

The only difference between the motor types is the position of the solenoid and therefore the battery cable runs.

A solenoid in the lower position on an Oka

My preference now is for the lower location since impact damage would be rare and obvious, rather than the side location (which is where ours is currently) where the solenoid is exposed to continuous exhaust heat, so I shall be changing mine for the lower type and fortunately I have acquired a spare frame of this type. For starters with the solenoid near the exhaust, I recommend fitting a heat shield of shiny aluminium, or even reflective tape, between them to reduce the effects of radiant heat.

Armature Problem

Well I've now done some analysis of the faulty armature.

I started by buffing up the commutator to ensure that the brushes would seat properly and eliminate that as a problem. I didn't have a lathe big enough to hold the armature so I made a simple stand by clamping the bearings on each end in small vices clamped to the workbench. I then turned the armature by hand and used a flat file to dress the commutator. [I did try using a cordless drill and belt to turn the armature but too many hands were needed to hold it steady and it only took about 10 minutes anyway].

Rudimentary lathe assembled to dress the commutator

Once the armature was smooth and flat, I rebuilt the motor and tested it on a car battery using jumper leads.

The motor still didn't turn but a whiff of smoke indicated where the problem was: 2 adjacent segments are presumably shorted internally and a burn mark was evident on the end of the commutator where the smoke appeared.

The burn mark where the smoke appeared from.

To confirm that this was not caused by internal short circuits in the area of the brush plate, field coil connections and/or fixing bolts, I removed the end cover and held the motor in a vice while applying voltage.

The motor still didn't turn but it put up quite a light show to confirm there is indeed an internal problem with the armature, irrespective of its rotational position.

A light show but no movement, and I was wearing eye protection.

Conclusion

So the armature would seem to be completely cactus and probably not worth repairing. But it also confirms that our problem in Eucla was indeed caused by 2 concurrent problems, the failed solenoid and an armature fault, although I now suspect that the armature fault was the primary show stopper.

The fixed field coils seem to be very robust so should be more reliable than rotating/sparking parts:

They don't move,
They're made of thick, solid copper bars,
I have a spare set.

Brushes can be replaced and I have a spare set, plus brush plate.

Solenoids can be replaced (I now have a spare), bypassed or even fixed on the side of the road but not the armature.

So the question now is whether to acquire a new or s/h (spare) armature or a whole new (spare) starter motor or get this one repaired.

Or do nothing and hope the existing one will last for ever.