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petter AV1 enging vibration

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petter AV1 enging vibration

Post by RTech on Wed Jul 16 2014, 13:35

Hi I have a petter AV1 engine that has vibration issue's at 1500 RPM We have fixed anti vibration mounts which have made things worse looking at most engine's they seem to be mounted on wooden blocks can anyone help me thanks Nigel

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Re: petter AV1 enging vibration

Post by Villiers on Wed Jul 16 2014, 16:45

What is it driving? these engines tend not to be balances as such and if all you have is the engine on the floor then it will vibrate, bolt it down and it will be better, if it is to be a display then slow it down if you can, unless it is driving a genny then that shouldn't be a problem.

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Re: petter AV1 enging vibration

Post by woody on Wed Jul 16 2014, 17:52

Bolt it to some heavy steel I beam sleepers.

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vibration

Post by RTech on Thu Jul 17 2014, 07:52

Hi thanks for your reply that was what i had in mind, we are going to use it as a test engine to try out a new type of fuel and it was running with out the vibration up to 1400 rpm but because its on nylon wheels on a shiny floor anything above that off she goes side ways,
Your reply should please the guys at work as we thought something was seriously wrong with the engine.
thanks again Nigel

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Re: petter AV1 enging vibration

Post by woody on Thu Jul 17 2014, 15:34

Any single cylinder engine, and plenty of two pots will have a degree of vibration, even if the crank has counterbalance weights.

With your current set-up you probably hit some resonance at 1500 rpm. Adding weight will always help - except your back when lifting it !

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Re: petter AV1 enging vibration

Post by Biggusdannus on Fri Jul 18 2014, 10:02

My AV1 was fairly lively at 1500rpm, I swapped the spring to get it to run at 800rpm for rally purposes. Much better!

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Re: petter AV1 enging vibration

Post by Darryl Ovens on Mon Jul 21 2014, 21:24

Engine vibration and mounting
Reciprocating machine vibration can be fairly complex.  However it can be broken down to make it more manageable.

(First a few basics;)
If you consider unbalance in a rotor with a horizontal axis.  (Ie a simple shaft between two bearings).  Looking down the shaft axis the unbalance will generate a force, the direction of which rotates with the unbalance.  If we look at only the horizontal axis (or component), it is a sine wave like force.  The same applies in the vertical axis.  The horizontal and the vertical forces are the same size, but one “leads” the other by a quarter of a turn or 90 deg.

How the machine and bearings respond to this force depends on how the bearings are “restrained”.  Note that usually the vertical and the horizontal mounting “restraint” is quite different, often the vertical axis is much more rigid than the horizontal axis.

There are three parts to the mounting restraint;  
One is how stiff the mounting is, ie how much force is needed for a given deflection, (like a spring).
Another is how much mass there is in and at the mounting.  The mass causes the response to be frequency (speed), dependent.  Think of it like this; at lower frequencies there is more time for the mass to start moving, so there is more response, (for the same force).
The third is how much damping there is.  This tries to reduce the deflection and absorbs energy in doing so.

At low speeds the unbalance force is low so there is little vibration, even though there is more time for the mass to start moving.  At high speeds there is more force but insufficient time for the mass to start moving.  
In between there is a speed range where stiffness and mass work together as in a pendulum (or swing), the deflection force due to unbalance gets the mass moving, but this results in deflection where the stiffness (like a spring), slows the mass and pushes it back towards where it started.  However in this speed range this occurs at roughly the same time as the unbalance force has also reversed and is now pushing it in the same direction.  Now there is twice as much force pushing the mass, so it moves twice as much, next time there is three times, then four times etc, so it builds up.
This is called resonance, and is made use of in pendulums and children’s swings etc, to over time produce much greater movement than you might expect from the force available.

Fortunately the third part; “damping”, is what prevents the vibration from continuing to build up indefinitely.  Damping is usually proportional to velocity, and is usually a mixture of friction and viscus damping.

In the speed range below resonance the rotor rotates around the centres of the bearings, and mounting stiffness dominates so deflection or “vibration” is proportional to speed and stiffness.  The angle of the deflection tends to lag behind that of the unbalance, this is due to the inertia, (caused by mass).

However in the speed range above resonance, the mass dominates and the rotor runs around its centre of gravity (CoG).  If the centre of bearings is not aligned with the rotor CoG (called unbalance), then the bearings also move around the rotor CoG.  Deflection is equal to twice the spacing of bearing centre to rotor CoG, but now it tends to be 180 deg to what it was when speed was below resonance.

As a rule of thumb;
Increasing the stiffness, increases the resonant frequency (speed),
Increasing the mass, lowers the resonant frequency.

Isolation mounts usually use this effect by lowering the stiffness and resulting resonance frequency well down below running speed.  Sometimes extra mass is added, (called seismic mass), to also lower the resonance frequency.

Reciprocating machines introduce a couple of complications.

For example; a horizontal crankshaft with a single vertical cylinder.
Looking at the vertical and horizontal parts, in this case:  
Horizontal unbalance is due to the crankshaft and only part of the con-rod.  Where as,
Vertical unbalance is due to the crankshaft, the con-rod and the piston.

There is now more mass moving vertically than there is horizontally.  This is a problem when sizing the counterweights.
If they are correct for the horizontal unbalance, then they are not big enough for the vertical axis as there is still the other part of the con-rod and piston that is unbalanced.  
Alternatively if they are correct for the vertical unbalance, then they are too big in the horizontal axis.
Short of the extra complication of balance shafts, the best that can be done on singles is some compromise between the two,  Hence the comment made previously that single cylinder engines will have a degree of vibration.

On (in-line) twin cylinder engines, the cranks can be arraigned so they run at 180 degrees, this counters much, (but not all), of the above unbalance.  There is still a rocking component as the two cylinders are not on the same axis, and a crank and con-rod does not quite give a true sine wave motion to the piston.  This is more pronounced for short con-rods and less pronounced for long con-rods, and causes smaller vibration components (if I remember correctly), at the odd multiples of running speed, which is why balance shafts often run at 3 times crank speed.

The most elegant solutions to unbalance in reciprocating machines are the opposed and Vee configurations.  
In the above example if you add another horizontal cylinder to the vertical then the counter weights can be made equal to the crank con-rod and piston weights and can now be nearly correct for both axes,  There will still be the odd multiples, but this is why Vee engines are so much smoother.

The best is the opposed configuration with 180 deg cranks.  Here all the components of one cylinder are countered by the opposite cylinder, especially if the cylinders are in line (not offset slightly as they usually are)and the cranks are symmetrical, (quite difficult to arrange).

Well that’s the theory anyway!

Now considering your example;
Is the excessive vibration mostly vertical, horizontal or “torsional” ie rocking around the crank or a mixture of these.  Torsion is more common in higher compression engines like Diesels.   Notice the heavy flywheel, does the engine have at least as much torsional inertia or was the designer relying on it's mounting providing some of this inertia?  Think about the flywheel pushing the piston up against compression, the engine inertia has to be sufficient to hold back against this rotation force, then the same in the opposite direction after the piston goes over TDC, more when the engine fires!
However torsional vibration gets worse at lower speeds and you will have noticed it gets extreme when the engine stops!
When you tried the “anti-vibration” mounts, were these a type of isolation mount, intended to reduce the vibration transmitted to the mountings?  If you reduce the stiffness you reduce the amount of vibration transmitted, provided that reducing the stiffness does not lower the resonance nearer to running speed, (or one of the odd harmonics).  My guess is this is what happened in your case.
It is possible that making the mounts even softer may solve your problem, (ie move the resonance to the other side of running speed), unless the mounting gets too wobbly or unstable.

Remember that adding mass will also reduce the resonance frequency so there is the same risk that this might also increase the vibration.

Is the floor rigid?
Ie is it concrete or wooden?  If wooden have you tried the engine on a rigid concrete floor?  Or at least directly over. or as close as you can get to the supporting piles or columns.

It is on nylon wheels, are some of these casters?  How springy are they?  It's possible that if they are springing vertically, either in the wheel or in the caster, or both, this will reduce the vertical stiffness and resonance frequency.  You could try temporarily blocking the engine frame up off the wheels with something really rigid, (like steel blocks), to see if this makes any difference to the vertical vibration.  The problem with this is that it will also change the horizontal stiffness so you will need to decide what the change is, in the vertical vibration only,  the horizontal could well get worse!  However if vertical is improved enough, then a solution may be to make the wheels much stiffer vertically.  You may have to change to steel or cast iron wheels, (perhaps with thin hard nylon tyres) and stiffer casters (if casters are fitted).
This may need to be tried on a hard rubber mat (say a hard door mat), or similar in case the (wheels or), blocks don't all take the weight evenly.  
Are the axes of the wheels parallel to that of the crankshaft?  If on castors have you tried turning the castors (ie right angles or parallel to the crankshaft), to see what difference that makes, if any.  This will dramatically change the horizontal stiffness, and horizontal resonance frequencies.

The above all assumes that the vibration is not such that a wheel (or wheels are) is not lifted off the floor (or in the castor/wheel bearing clearances).  When this happens the stiffness becomes “non-linear” and all sorts of weird things happen, as things bounce against the floor or within the clearances.

There was an idea that diesels should not be mounted hard onto a concrete footing.  I'm not sure if this was for shock “absorption” or perhaps the wood has enough “give” to take up an uneven mounting surface and prevent a condition called soft-foot, where three feet are solid but the forth has a gap under it.  I have not heard mention of the wood idea in current times, till now, and cannot comment further, other than it would reduce the mounting stiffness slightly.

If a vibration meter is available, I would measure the vertical mounting resonance frequency, using bump tests, to see where it falls relative to running speed, then you know which way to try to move the resonance, to get it well away from 1500 rpm, (25 Hz).
Also a vibration meter will allow you to measure in one axis and put a number on the levels, this makes it possible to compare the levels in separate runs.  If you don't have access to something that can measure (or at least indicate) vibration, you can separate the axes by lightly holding one end of something like a light pen or pencil, in the direction (horizontal or vertical) that you are trying to determine, and hold the other end against the machine.  You won't feel vibration at right angles to the pen anywhere near as much.   Avoid holding it against springy sheet metal, and use some rigid part like castings, ie the head or crankcase.  If you are checking radial vibration choose a place so that an extension of the pen axis passes through the centre of gravity of the whole machine, for horizontal usually just above the level of the crankshaft, for vertical the centre of the cylinder head above the crank.  If trying to check for torsional vibration choose something rigidly attached and as far away from the crank as possible and tangential to it.  On this case vibration felt should reduce, (maybe down near 0), as you get closer to the centre of gravity.

Having said all the above, it may still depend on the Horizontal/vertical balance compromise chosen by Petter designers.
 
I have seen flywheels that have been out of balance, even new ones!  
And twin flywheel stationary engines where the flywheels were put back but reversed, putting the counter weights nearly in line with the crank big end!

Sorry this turned into a "novel" it happens to have been "my field" at one time.   Smile 

Cheers
Darryl

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Re: petter AV1 enging vibration

Post by RTech on Tue Jul 22 2014, 08:18

Cheers Darryl looks like you've spent most of your spare time fiddling away on stationary engines, I understood a fare amount of what you have said and I'm in the process of removing the anti-vibration mounts from there present position fixing the engine back to the original frame where the vibration was minimal and building an additional frame bolting it to the floor and repositioning the anti-vibration mounts between this frame and original frame which hopefully will hold and absorb the sideways and vertical movement.
Thanks for the awesome reply Darryl,
I will post again later on this week and give you the results.
Regards Nigel

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Re: petter AV1 enging vibration

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