So a couple of months ago my prop snapped on the way to Le Mans caused by prop adaptor coming loose (bike engine). After eventually getting the car
back in the country, and then getting my prop sorted by Dunning & Fairbank (new front section with stronger steel, new slider, new centre bearing
etc) I've put it back together and gone for a short test drive. There was an odd knocking when setting off (not even particularly aggressively)
so I've done some more investigation by way of video camera. 1st video in slowmo looking down at the prop in the tunnel here: -
And watching that in real time scared the bejesus out of me!! Second video is the same time but from a camera mounted near the engine looking
rearward: -
You can see that it wobble like crazy in the rubber mounted centre bearing. Don't worry about the very bottom if this video where it looks to
be offline, that' just one of the balance weights. When it wobbles though you can really see it moving in the centre bearing, and you can see
the rear section of the prop moving too (the white bit as it's in light).
So the question is, what the blue buggery would cause such violent movement, and would you expect any movement there? Is it a combination of
heavier grade prop and possibly "softer" rubber centre bearing? Is it alignment (which I can't do a great deal with, the engine is
pretty much where MK designed it and the same in all). Is it something else? Everything is fastened securely, and when I turn the prop manually when
up on axle stands all seems fine, it's just when under load.
Ideas please? I can't drive it as it is and it's sunny! (at the minute...)
Photo Archive
Building: Built v8 striker/blade fury/ R1 westy,now zetec we
posted on 22/8/15 at 03:35 PM
Props not too long now is it and compressing the slider to its limit. That would cause it to want to try to shorten itself by rotating out of square
at the centre bearing.
The problem is the shaft is "whirling" because it is operating in the RPM range close to its critical frequency.
By making the shaft "stronger" I take it to mean they re-made the shaft the same diameter but using thicker wall thickness tubing, this
may well have made the problem worse by increasing the mass of the shaft.
AS with most vibration issues the solution is to change the design to move critical frequency out of the operating rpm range.
The variables are the mass and length of the shaft relative to its' stiffness.
Looking at the videos I would make an educated guess that the long section of shaft is too long relative to its stiffness
On RWD road cars on poverty spec and vans models the manufacturers would use a single prop piece shaft made of large diameter but thin wall tube, on
more powerful up market models a 2 piece small diameter shaft made of thicker wall tube would be used with each section of shaft fairly short.
[Edited on 22/8/15 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
Whirling will start a speed approaching critical speed and greatly increase in amplitude the closer the RPM approaches the critical speed..
How well the shaft is balanced is not a factor there will always be enough asymmetry, static deflection or out of balance mas for or vibration to
trigger the phenomenon the amplitude and the frequency at which the resonance occurs is a function of the mechanical properties and dimensions of the
shaft.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
Thanks all, quite a few different things to look at. Let's see.
Phasing - the phasing I believe is correct, it's certainly what it was previously i.e. yokes aligned as per this link:
Linky Dinky
A couple more videos whilst it's up on stands (wheels and drums off so minimal load): -
When I blip there's minimal noticeable deflection (thought there is some when then wheels are on and there's a load there). Only thing I
can really see is in the 1st video the front prop section looks to be slightly untrue at idle (but I recall similar in the old one and I didn't
have the same symptoms). As it's been balanced (as had previous props) I can only assume it's actually in balance and maybe a feature of
the angles?
Slider : I've removed the prop from the adaptor and there's still > 15mm of slider left so I don't think it's too long.
Also slides freely. Makes sense though, really thought that was it!
Don't think there's any wear at the output. Certainly heavy "tugging" on the prop by then engine reveals no play at engine
end.
Re the whirling/ critical frequency - is that likely at low speeds? The videos are at around 10-15mph. Fag packet calculations tell me rolling
circumference of my 195/50R15 is about 1.75m, which is about 919 rotations per mile or 153 rotations per mile at 10mph. Depending on diff ration (not
sure what mine is, but will be between 3.18 and 3.92) that's 5-600rpm at the prop so a long way from the x000 in those calculations?
quote:Originally posted by mark chandler
My prop wobbles and shakes a lot more than that, as it's just when you pull away not when booting it down the road it could just be normal
behaviour.
Has it got any marks where it's been hitting things, prop catcher perhaps?
Haven't tried it at speed, this low speed wobble has put the willies up me so want to understand the cause before I spank it Dont believe the
prop is hitting anywhere, just the centre bearing moving to the extremes if the housing. Guess it's possible it used to do that with the old
prop to a lesser extent but never had reason to watch the prop whilsy driving.
So I went for a short drive this afternoon after re-fitting both ends of the prop in case anything wasn't fastened down correctly (it was all
fine).
Wobbling still present, but I got slightly braver and took it up to around 60mph, and it seems that once it gets past that initial shaking around
10-15mph it smooths out so might be OK at speed, but I'd want prop catchers just in case (I plan to make some anyway) and the issue at
low speed still it not acceptable.
So it seems similar to the whirling that BT mentioned but the speeds didn't add up, but this describes whirling of a 2-piece prop under load at
low speed (as well as lots of theory I don't really get!) I assume I'll probably get harmonics at different speeds too.
Long and the short of it, it seems that it is a combination of heavier prop (TRT rear which I've had a while, and stronger front section) and
presumably less damping on the rubber mount, so only a redesign is going to sort this, and I've thrown a couple of hundred quid away on a strong
but wobbly prop!
Going to ask D&F if there's a sturdier centre bearing they can do, otherwise I may just go back to basics and get a stock MK unit and put it
down to experience
Diff flange and gearbox output/flange at the same angles? (ie, parallel to each other in all planes) or at least close, otherwise that can have the
same effect as the uj's being out of phase and cause all sorts of vibrations and wobbles.
quote:Originally posted by Oddified
Diff flange and gearbox output/flange at the same angles? (ie, parallel to each other in all planes) or at least close, otherwise that can have the
same effect as the uj's being out of phase and cause all sorts of vibrations and wobbles.
Ian
No, because in the MK the engine is at an angle to fit in - same with all bike engine MK's. However it's no different to how it was
before the prop was modified, and it didn't whirl then. Think it's just heavier and less damped.
Photo Archive
Building: Hatred of Loughborough's Speed Humps
posted on 25/8/15 at 08:33 PM
Is there a TRT tube section?
Presence or absence of the TRT may make a difference? A worn TRT certainly will....
I too have had wobbles/noises/vibes at very low paddock speeds, but the operating environment on the track (and road to some extent) requires 10K+ rpm
from the engine. No issues at speed.
Same happens with the current chain drive too, it'll slap and rattle at insensible speeds, but it's all fine at operating rpm.
Dont believe the
prop is hitting anywhere, just the centre bearing moving to the extremes if the housing. Guess it's possible it used to do that with the old
prop to a lesser extent but never had reason to watch the prop whilsy driving.
