speed wobble

[C00kiemonster]

What about suspension? Have you checked both forks for settings? Is one much softer set than another? Has the preload on one of them slipped or been changed ?

[Monty]

Can't be wheel balance as it wouldn't stop as you go faster. It would get worse and worse as speed increased.

Not sure what it is though, trying to think when I saw something similar as its ringing a bell

Not disagreeing at all Monty as I tend to agree but at work when we are spinning up rotating machinery its some times in hi alarm for vibration and if we speed it up the vibration reduces and this is after we have already driven through the critical speed zones.

Could it be same with wheel balance ? You know a lot more than me on this subject and are better technically so was hoping you could answer as I m not sure what actual process is happening.

Exactly why I thought weights first Blade. All large rotational equipment I've worked on has had a critical speed for unbalance, plus I've had cars with a wheel shimmy that is only in a specific speed range and it always turned out to be wheel balance.

But not to say if something was wildly unbalanced it wouldn't do what Monty suggests.

Vibrations in rotating machinery will more likely be axial vibration from misalignment (e.i. along the axis). An unbalanced wheel will always be a radial vibration and the only way it will stop is if the centre of gravity is moved back to the centre of the axil. Increasing the speed will not stop it, and can only increase it. F=ma.

Actually thinking about it what are your bearings like Chris?

[Blade]

We have a scenario at work in which radial vibration decreases when the rotational speed increases and as said previously this is once the machine is above critical speed zones where the vibration trip is inhibited. We monitor two radial channels (x & y) on each bearing housing and the axial position, but not axial vibration and have not personally any experience of axial vibration. I fully accept it is very weird and goes against conventional theory but I can promise you this happens. Next time I see the rotating equipment technical authority engineer I will ask him to explain as this is baffling me and something I previously just accepted but would now like to know why.

[Monty]

Then what ever your rotating machine is its doing more than just rotating like a wheel, what is it a turbine? Must have some other dynamic force on it

[Blade]

Not sure mate. The machine in question is a Centrifugal Compressor. We have boroscoped the internals and the cause of vibration is salt crystallising on the impellers.

[kiwikrasher]

We have a scenario at work in which radial vibration decreases when the rotational speed increases and as said previously this is once the machine is above critical speed zones where the vibration trip is inhibited. We monitor two radial channels (x & y) on each bearing housing and the axial position, but not axial vibration and have not personally any experience of axial vibration. I fully accept it is very weird and goes against conventional theory but I can promise you this happens. Next time I see the rotating equipment technical authority engineer I will ask him to explain as this is baffling me and something I previously just accepted but would now like to know why.

Yup Blade, same experience, both with Gas Turbines on jets while in the RNZAF, Steam Turbines and Power Recoveries (Big f*ck off turbo's basically) at the refinery. If I remeber my Trade training right, it's more a resonance vibration due to the fact the shaft bows under increased acceleration as the rotating mass imparts a outwards force on it, and for some reason that hits a peak at a certain RPM or as you and I know it "critical speed" and then settles down. Why it behaves like that I can't remember, I did my trade training in 92 and it's just been one of those "knowns" I've also accepted since then!

[kiwikrasher]

We have a scenario at work in which radial vibration decreases when the rotational speed increases and as said previously this is once the machine is above critical speed zones where the vibration trip is inhibited. We monitor two radial channels (x & y) on each bearing housing and the axial position, but not axial vibration and have not personally any experience of axial vibration. I fully accept it is very weird and goes against conventional theory but I can promise you this happens. Next time I see the rotating equipment technical authority engineer I will ask him to explain as this is baffling me and something I previously just accepted but would now like to know why.

Yup Blade, same experience, both with Gas Turbines on jets while in the RNZAF, Steam Turbines and Power Recoveries (Big f*ck off turbo's basically) at the refinery. If I remeber my Trade training right, it's more a resonance vibration due to the fact the shaft bows under increased acceleration as the rotating mass imparts a outwards force on it, and for some reason that hits a peak at a certain RPM or as you and I know it "critical speed" and then settles down. Why it behaves like that I can't remember, I did my trade training in 92 and it's just been one of those "knowns" I've also accepted since then!

Just Googled "resonance" and got this.. I've got too much on tonight to nut it all out but this'll makes sense to Monty I guess!

In physics, resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies, or resonance frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.

Resonance occurs when a system is able to store and easily transfer energy between two or more different storage modes (such as kinetic energy and potential energy in the case of a pendulum). However, there are some losses from cycle to cycle, called damping. When damping is small, the resonant frequency is approximately equal to the natural frequency of the system, which is a frequency of unforced vibrations. Some systems have multiple, distinct, resonant frequencies.

[kiwikrasher]

OK, managed to do a bit more reading and as I did a bit more from my training came back to me.

This is how I understand it, not gospel.

In simple terms, with rotating equipment like turbines, centrifugal compressors and the like, the material the shaft is made from has a natural frequency, as in if it is deflected from its axis, the rate of osillation as it works it's way back to static and it's axis in line. This reducing deflection is it's damping.
As you accelerate the rotation speed (start up) the rotating mass deflects the shaft, the shaft will try to dampen causing osicallations. When the rotational speed matches the natural frequency of the shaft, the damping effect is negated and oscillations will be at their maximum, and therefore rotational vibration at it's maximum, this is usually critcal speed.
Now due to rotational speed and natural frequency likely in some cases to match a few times there can be more than one critcal speed.

Why Chris's wheel wobbles, I have no idea

[Monty]

OK, managed to do a bit more reading and as I did a bit more from my training came back to me.

This is how I understand it, not gospel.

In simple terms, with rotating equipment like turbines, centrifugal compressors and the like, the material the shaft is made from has a natural frequency, as in if it is deflected from its axis, the rate of osillation as it works it's way back to static and it's axis in line. This reducing deflection is it's damping.
As you accelerate the rotation speed (start up) the rotating mass deflects the shaft, the shaft will try to dampen causing osicallations. When the rotational speed matches the natural frequency of the shaft, the damping effect is negated and oscillations will be at their maximum, and therefore rotational vibration at it's maximum, this is usually critcal speed.
Now due to rotational speed and natural frequency likely in some cases to match a few times there can be more than one critcal speed.

Why Chris's wheel wobbles, I have no idea

I put 2 screw compressors on the roof of a well known Auditors in London and had major natural frequency problems. Managed to hit the exact frequency of a beam that ran down through the entire building. They had people going off sick with blurred vision and nausea for weeks before we found the problem.

As these were fixed speed compressors we solved it by changing the length of the inlet and discharge lines which moved the frequency away from that of the beam.

[Kwacky]

But does the wheel still wobble?

[Frankie]

Have you got to the bottom of this yet?

[rocket]

I have the same problem on the black bird , it seemed to do it when I have my top box on

[Blade]

You tried taking the top box off ??? You don't need that many sandwich's

[Monty]

I have the same problem on the black bird , it seemed to do it when I have my top box on

That's where I've seen it before, my f650 did it with the top box on

[Deegee]

Does a top box create drag above the CoG which via leverage lifts the front increasing trail and lessening load over the wheel? More trail normally stabilises steering, but if the wheel is only carrying a partial load it might happen I suppose.

[Monty]

No, I think it's just magic

[kiwikrasher]

No, I think it's just magic

is that your final professional engineering submittion?

thinking along the resonance line, I wonder if it's a unbalanced wheel acting against an under dampened front shock set up? Natural frequency and spring frequency could possibly yeild similar results.

[Monty]

No, I think it's just magic

is that your final professional engineering submittion?

thinking along the resonance line, I wonder if it's a unbalanced wheel acting against an under dampened front shock set up? Natural frequency and spring frequency could possibly yeild similar results.

No that's just what I say when I exceed the point of my understanding!

We could test this theory, if Chris was to take off the wheel weights and go for a spin. Might either stop it or make it happen at a different speed.

[kiwikrasher]

We could test this theory, if Chris was to take off the wheel weights and go for a spin. Might either stop it or make it happen at a different speed.

I wonder if he minds being our test dummy to satisfy our curiosity?

I think i'd prefer him to get it balanced and then try. Otherwise we are going from a unknown balance condition to an almost guaranteed unbalanced condition. That we can find out if it was unbalanced to start with and see the result of it balanced.

Or maybe it's already sorted and he's just laughing at us trying to be clever

[chris]

right ive decided to order some new bearings anyway, when I take the wheel to change the bearings im going to get it balanced aswell,

ill quite happily try taking the balancing weights off to see what happens I wonder how much difference it will make