Brake issue?

[DrZoos]

So ever since we got plane new left wheel makes a slight doonk doonk doonk as it turns...after investigation by a LAME it was just the floating calipers and disk alignment

 

It just got noisier today and wheeling plane backwards into hanger the brakes got gradually harder to the point i couldnt pull it...brakes where off.. I jacked up plane , did nothing put it back down and all was good again...

 

I had spat off prior to last flight as a fellow pilot had said he thought one brake worked much harder than the other...

 

There is nothing visually wrong we can see...

 

I will pull it all apart and have another look sunday or monday...

 

Any suggestions or input on what might be an obvious cause is appreciated.

 

 

[JEM]

Its not a wheel rim bolt loose and touching the brake as the wheel revolves is it?

 

 

[cscotthendry]

DrZoos:

 

Although I'm not having brake lockups, I have been chasing a clunking noise in one of my wheels. Yesterday, I discovered that when I swapped the wheels, the noise went with the wheel. It had nothing to do with the brakes at all. I further discovered that the outer bearing in the offending wheel will fall out of the hub when the wheel is laid down with the outside face down. From this I have deduced that the bearing is moving in the hub as the wheel is rotating and is causing the clunking noise.

 

My first plan of action is to secure the bearing in the hub with some Loctite bearing retainer compound. If that doesn't work I will replace the wheel.

 

 

[DrZoos]

Thanks guys i will check both. At this stage any suggestions are great and i will check them all...really appreciate these and any others...

 

Would really like some input on the brakes as well.

 

 

[JEM]

What aircraft type is it?

 

 

[DrZoos]

alpi pioneer 300

 

 

[Head in the clouds]

Floating disc/fixed caliper brake systems are compact, light and quite efficient but they are not without their issues if not maintained carefully, or especially if not installed correctly.

 

There are several different brands and designs and you don't say which you have so it's difficult to offer any specific advice.

 

They are well known for for making one-per-rev clunks while taxying, if it's minor you can usually stop it temporarily with a tap on the offending brake pedal but if it's getting noisier it may be an indication of a warped rotor (though overheating perhaps) or wear where the rotor engages the hub, sometimes this is via lugs on the rim at outer perimeter of the rotor and sometimes at the inner perimeter or via pins protruding axially from the hub. So if it's getting worse looking for wear might be your starting point, if it was always bad then looking for poor installation might be the go.

 

A few years ago I fitted a set of FBI Black max wheels and brakes and I followed the installation manual to the letter but was then rather concerned to discover that the even the manufacturer didn't understand how their own brake system functioned because they hadn't made any allowance for the brake pads wearing over time and there wasn't sufficient space for the rotor to float toward the fixed pad as it wore. Instead the rotor reached the end of it float travel and had to warp as the brakes were applied. Needless-to-say this would cause damage to and/or failure of the rotor or caliper over time, and certainly reduce the effectiveness of the brake.

 

At the time I wrote up the issue on the build log on another forum, so I've pasted it below in case it might be helpful -

 

A few observations about Black Max brakes, having finally got them working yesterday.

 

 

 

The quality of the kit of parts is really excellent but there were a few disappointing aspects that really do need mention.

 

 

 

I have the 6" heavy duty split rims and HD tires also.

 

 

 

My first disappointment was the bolts supplied to connect the two halves of the rims, they are not the kind to use as the'grips' on the underside and the lock washers would damage the aly rims with each assembly/disassembly so I had to find suitable ones and as nearly everything is metric here that was a difficult exercise. Eventually I found good Socket Head Cap Screws (SHCS) which I favour anyway, so that was OK.

 

 

 

I followed the assembly instructions carefully and that's when I noticed the first really bad (IMHO) engineering defect and that is the concept of running the bearings without proper control. Their idea is to have a cantilevered axle fixed to the airframe, slide the wheel and bearings onto the axle and tighten the nut until the bearings are being strained so the wheel can't rotate and then loosen the nut off until the bearings are sufficiently unloaded to turn again. That is not the way to do things, if for no other reason than that it then relies on the stiffness of the nyloc axle nut to prevent the nut position changing towards tighter on the right and looser on the left. Their axle is drilled for a split pin but in that case a castellated nut should be supplied, and it'd still be a bad setup. And also, whether the bearings turn on the axle or not is dependent on a tight fit of the bearing on the axle, in which case it invariably will gall the axle and make it nigh impossible to remove when required.

 

 

 

The other reason it is a bad move is that it precludes the use of a more normal bolt-type axle where the bolt can be torqued properly and it is then that tension which holds the axle level and firm. To use that method a spacer tube must be employed where the spacer either goes right through both bearings or alternatively is installed between the bearings and form a compression sleeve. The latter is much better but it must be accurately machined to length within a couple of thou so that it then controls the bearing preload properly and also prevents the bearing inner from spinning on the axle (or sleeve, in the former case).

 

 

 

My wheel mounting is arranged so that the wheel effectively sits in a clevis and a bolt goes right through from one side to the other and is then tightened, just like most nose wheels and many mainwheels. Strangely the Black Max system doesn't allow for that at all unless you are willing to leave the axle bolt loose which I couldn't do even if I was willing to because my 'clevis' is actually a pair of gear legs, one each side of the wheel, DC3 style. If I didn't tighten the bolt the legs would 'walk' and allow the wheel to be steered by the castoring angle. That could get very exciting in a crosswind!

 

 

 

So the second thing I had to do was machine up a pair of precision bearing spacers and find imperial bolts to use as axles. I'm not complaining, I know planes can have their differences but my style is the more common and theirs is a rarity, at least I've never seen anyone do it that way before.

 

 

 

Continuing to follow the assembly instructions I got to the stage of fitting the floating discs (rotors), and I love floating disc systems, so light and efficient, even if they do clatter while taxying if not adjusted properly. I put them together just as the instructions required but something just didn't seem right. It took me quite a while to work out what it was.

 

 

 

Three holes in the disc sit over the heads of three SHCSs which are screwed into the hub and that is a fine arrangement. The actual assembly is by fitting the slip-in brake pads in to the caliper then place the rotor between them and hold the caliper up against the hub while sliding the axle through the assembly. So then the brake caliper sits up against the hub bearing the disc will be fully onto the SHCSs which locate the disc i.e. the heads of the SHCSs will be protruding as far as possible through the disc, on the caliper side.

 

 

 

Unfortunately that is a serious mistake because there is no adjustment available for when the pads wear. Consider that the inner and outer pads wear at the same or very similar rate and the caliper cannot move along the axle relative to the hub because it is held between the hub and the clevis/nut/whatever. Only the outer pad can move, made to do so by the brake piston i.e. it is a single piston system, not a twin piston system (floating disc, not floating caliper). Therefore as the pads wear the floating disc must be able to move towards the fixed pad by the amount of the wear of that pad. So - when the pads are new the disc/rotor must be aligned with the outside (not the inside!) of the SHCSs on which it floats, so that it can move toward the hub as the pads wear.

 

 

 

The way Black Max describe the assembly the disc and pads would be fighting each other as soon as any pad wear began and brake effectiveness would be lost and damage to both pads and disc would be certain.

 

 

 

The fix was simple, about 7/64"/3mm of washer between the hub and caliper allows enough motion of the rotor until the pads need replacing.

 

 

 

Bleeding the brakes was a trial also and can be made much easier. The 'master cylinder' is a tiny chamber just forward of the brake handle (I have the single lever on the joystick type). The idea is to remove the grub screw which is effectively the brake fluid reservoir cap (don't use brake fluid, only hydraulic oil, power steering, auto trans type) and bleed as per a car/motorcycle but top up the fluid every two pumps. I started like that but it quickly became clear that it would be a hundred or so top-ups before I had fully purged the system of air. So I machined up a plastic fitting to screw into the 'reservoir' and which had a barb on it to accept a plastic tube which I connected to an inverted bottle of fluid hung from above. A simple solution and a fitting which would be very easy to be supplied by the factory, instead it took me ages because the thread is 16TPI which my metric lathe doesn't accommodate easily.

 

 

 

One last gripe - the brakeline fittings to the caliper that were supplied were the straight type, there was no mention of an elbow type until I contacted the dealer here when having to replace the one I damaged. If they'd mentioned elbow types when I ordered them, that is what I would have bought (as most people probably would) but instead I had to pay 5x the price to get them here.

 

 

 

OK all complaints over and the system, finally working, is excellent and a bargain at the price, all the brakes hardware, HD wheels and tires, freight to the door in OZ, all for under $1000.