F355 Aircon Recirculation Flap

Just got my new motor for AW. Made by MES Switzerland. Flap works perfect now. I knew the flap didn't work when I bought my car. PPI reported it. In fact, I'd have to say the PPI was very thorough. Anyway, the broken gear in my case, or the broken shaft in yours, may well be due to age. Plastic may get brittle with age and break. These motors failing seem to be an issue with many cars, VWs, Saabs, Audis,...

 

Impressive!

 

Very nice!

 

Wow great work,,, impressive

 

Wow! You guys are terrific. Is the 'continuously powered a/c re-circulation flap' a problem on all model year 355s?

 

@BLUESL

Is your wonderful solution, something you will make more of for sale, or alternatively would you post the details as a project.

 

Nice design work but I have to ask, why do you feel it is necessary, and what is the cost?

 

Very nice design.

 

To be honest, I'm not sure this is a problem that needs fixing. Plus, compare your $70 plus time and labor to assemble and install against a new motor/gearbox for $115. (Probably cheaper if you can find the source of the after market part. The seller I bought mine from removed the part number and manufacture name.) Motor failure doesn't seem to be the common problem, rather shaft and gear failure. The gears/shaft will be loaded by the dynamic pressure of the air on the flap when closed, increasing with speed, regardless of whether there is a potential across the motor. So, I'm not sure cutting the current would prevent gear/shaft failure in the long run. I would imagine most failures are due to the plastic parts becoming brittle due to age.

Please don't take offence. Just my opinion. I still respect the design.

 

Brilliant solution, beautifully executed, BLUESL. The factory should have done something similar to begin with, or at least should have fitted limiter switches. Your posts are an excellent example of what makes this place great. Thanks for sharing your knowledge.

For those of us who'd like to think that we could muster the soldering skills (I've replaced a few SMDs over the years) would you be willing to share the schematic and component list? Or heck, make them and sell them (although I expect the time you spent would necessitate a fairly hefty price tag, but maybe doing a printed circuit would save enough time to justify building a few). A kit would be fine by me. It's just that now that I've seen a truly elegant solution to the problem I won't be able to sleep until there's one in my car.

Again, well done. Cheers.

 

Here's why I think cutting the current at the flap stop positions is necessary:

The motor, even quite small, develops quite a high torque on the square shaft when stalled due to the multiple gears (significant torque multiplication). The torque is also quite high on the second last gear, pointed at by a red arrow on the pic in the earlier post. So, either the rectangular shaft or the mentioned gear will fail after some time.

I installed limit switches on my flap motor and it works perfectly (the new motor will last longer than the car, I think). Another good solution would be to insert an overcurrent sensing device that would cut the current to the motor before it develops the full stall (500 mA) torque, say at 200 mA. This should be fine as it is twice the normal running current of 100 mA so there should be no false cut-offs. I believe that "Bluesl's" device functions in this way.

 

I don't disagree with anything you are saying. I would only question what the peak current level needs to be to ensure that there is sufficient torque to close the flap and crush the gasket sufficiently so that it remains closed under the dynamic pressure on the flap when the car is under speed. And since the foam gasket is crushed, even after the current is cut, there will remain a torque on the gear set by the dynamic pressure applied to the flap and also by the force generated by crushing the gasket trying to push the flap open. There is sufficient friction in the gear train, particularly with the worm and wheel drive, to prevent unloading to the gears. Thus the stress on the shaft and the second gear that you refer to will always be present. It may be less than that which is present at the stall current level, but there will always be a significant "preload" on the gears and shaft which is required to keep the flap in the fully closed or open position.

Again, my comments are for the sake of discussion.


P.S. Also, owning a spider I'm not too concerned about whether the flap works or not. I'm pretty much getting fresh air all the time.

 

With the worm gear at the motor shaft, there is no way the motor will rotate back and allow the flap to move back under any pressure (it will brake). Furthermore, there is a number of other gears between the flap and the worm.

There are two things that cushion the mechanism - the gasket at the flap and the rubber mounts at the motor. A small move-back when the motor is stopped at closed position will be allowed by the motor mounts but there will still be some pressure at the flap gasket which, even if very small, will seal as the motor gears will not allow any further move-back. So, I wouldn't worry about the RAM pressure. After all, there is no harm if the flap does not seal perfectly; a tiny amount of fresh air will not really affect the air-conditioning.

 

Anyone got a good photo of the flap "gasket"?

 

You guys are really smart!

I love minutia and details but I'm totally lost and impressed.

Please carry on.

 

Expanding on this. If the system is fully elastic there would be no push back or unloading by turning off the current, assuming (correctly) that the worm gear does not rotate once the current is cut. Unloading would come from inelastic components. For example, if the flap gasket continued to crush (deform) under the closing force after the current ceased. This crushing would allow the flap shaft to rotate slightly, unloading the gear set. Inelastic motor mounts could have a similar effect as the entire motor case could rotate slightly relative to the flap shaft. The point is, there would have to be some (small) rotation of, or relative to, the flap shaft from its stopped position to unload the gears. This could be easily checked by closing the flap and placing a dial gage set to zero on the flap, furthest from the pivot axis. Then turn off the ignition to cut the current and see if the dial gage indicates any motion of the flap. The same could be done for the motor by placing the dial gage to measure its position. And the loading/unloading of the flap by speed dependent dynamic pressure would still generate additional loading of the gear set. It's not about leakage.

 

I have to correct myself about the above - The motor mounts will not allow any move-back of the flap when the motor is stopped. The motor mounts will be "spring-loaded" by counter-twist of the motor when the flap closes; on motor disconnect, as there will practically be no relaxation in the gear set, this "spring-loading" of the mounts will act on the flap and keep pressure on it. The compressed elastic flap seal (rubber foam sheet on my flap) will also keep pressure on the flap from the "other end".

 

Is the value of the over-current that the software is looking for pre-programmable? Can you, for example, select that the switch-off is at say 200 or 300 mA to avoid going up to the full stall (500 mA as you informed) and avoid exposing the gearbox to the maximum stress (even though for a brief period of time)?

 

BlueSL original said:

Now you say:

Ok, so it seems we now all agree that once the flap is closed there is no significant unloading of the gear train. So, it remains stressed to whatever level it was at the moment just before the current is cut off. Since Bluesl stated that his device allows for 2 seconds of over current before cutting off, we can rightly assume that the gears will be loaded to the 500 ma level, same as if the current remained on. For simplicity sake we can think of the system as being similar to torqueing a bolt. You use a torque wrench (the motor) to tighten the bolt to some torque value to achieve a specific clamping force (gear train load). Once you reach that torque, you can hold it forever (current remains on), or remove the torque wrench (turn off the current). Either way the clamping force (gear train load) doesn't change. Thus, the only roll cutting the current off plays is to eliminate current flowing through the stalled motor, and possible motor burn out. As I stated before, this doesn't seem to be a common mode of failure, so I don't see it as problem that needs fixing (post 36). The gear train remains stressed and failure of the plastic gears or flap shaft will remain the source of failure.

Additionally, consider the dynamic pressure that some have dismissed. At 100 mph the dynamic pressure is about 12 psf. The flap is 4" x 6"/144 = .17 sf. Thus at 100 mph the force on the flap due to dynamic pressure could be has high as 2 pounds, probably less, but still an additional load on the flap which is transmitted to the flap shaft and gear train.

So, is the original design bad or not? Depends on how you look at it. It appears to me that cutting off the current will have no significant effect on the observed, most common failure mechanism, and no significant impact on motor life, but saves the cost of more advanced circuitry in manufacture of the control unit, spread over many cars. Remember, Ferrari just buys an AC controller and flap motor from a supplier and that supplier is likely supplying similar units to any number of auto manufactures. Thus the supplier's cost savings could be significant. $0.50 over millions of units adds up. Perhaps for a purest's point of view it's bad engineering but from a real world engineering approach, with economic considerations, not so bad after all. If there is a design flaw it would appear to be in the choice of materials for the shaft and gears in the flap motor assembly. On the other hand, again considering that this flap motor may have ben installed in may different cars, Fiats, VWs, Volvos, Mecs, etc, (who knows?) perhaps we just see gear and shaft failure because Ferraris have a much longer life expectance than ordinary automobiles.

Anyway, as I said, this is just my opinion and comments for the sake of discussion.

 

Have not seen a basic program in 25 years - that was a cool flashback

 

Calculating the wind pressure on the fresh air flap would be rather complicated. When moving at speed, the air pressures over the bonnet vary from negative (suction) to some positive pressure which peaks at the base of the windscreen. However, the said peak pressure is much smaller than the RAM pressure on surfaces at the right angle to the air flow.

Depending on where the fresh air intake grill is positioned on the bonnet, it could be under suction or at some pressure. On the 348, the intake grill is not quite in the highest pressure zone but somewhat forward, towards the transition from pressure to suction. The pics below illustrate how the air flows over the car body (Cp - Wind Pressure Coefficient).
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