calculate the force on a big end Bearing

[Downunder]

As the heading states, I need to calculate the force in newtons of a big end conrod bearing onto the crank pin.

 

Is there an online calculator, or formula?

 

Would the same force apply to the piston pin bearing.

 

Obviously the force of the piston pushes the rod down and then the counterweght brings the piston back up(single cylinder) so there maybe a calculation for average and maximun force?

 

Probably not as simple as that?

 

The engine would be two stroke petrol with oil mix.

 

I should say this is part of an alternative bearing material investigation and will not be used in a man carrying aircraft.

 

Any help or tips would be appreciated.

 

Regards,

 

R J Mitchell

 

 

[Yenn]

No idea how to do it but I have seen it stated that the greatest loads are in the reversal at Bottom dead centre rather than at TDC

 

 

[facthunter]

Loads.

 

That is not correct Ian as ,the piston changes direction (reverses) in a shorter time near TDC, than it does at bottom dead centre. Connecting rods fail in tension at high revs when inertia loads are at a maximum, usually when the throttle is closed to change gear. Nev..

 

 

[facthunter]

amend.

 

Sorry downunder, you said it was a 2-stroke. In that case there is always a force on the top of the piston.. A ball-park figure would be to add the con-rod + piston assy weight and calculate the centrifugal force with it acting at the crankpin centre, at max revs likely to be attained. I can't help you with anything better. Nev..

 

 

[crashley]

force = mass times acceleration

 

mass = piston and conrod weight

 

acceleration = zero to max piston speed in x seconds

 

why do you want to calculate this

 

 

[hiperlight]

Sorry downunder, you said it was a 2-stroke. In that case there is always a force on the top of the piston.. A ball-park figure would be to add the con-rod + piston assy weight and calculate the centrifugal force with it acting at the crankpin centre, at max revs likely to be attained. I can't help you with anything better. Nev..

]Nev,

 

Please define and quantify 'centrifugal force'.

 

Bruce

 

 

[facthunter]

Define.

 

In simple terms a "flinging" effect resulting from rotation. the formula is from memory , mass times velocity squared over Radius. There is another that uses radians, but gives the same result. Strictly speaking, we could be more precise and talk of centripetal force as well as centrifugal force( one acting opposite and equal to the other). I try to keep it simple. Nev..

 

 

[hiperlight]

Nev,

 

I don't think it is quite that simple...'centrifugal force' is an apparent force...centripetal force is a net force applied to an object to change the object's straight line inertial path which is tangential to its new circular path. Without a centripetal force (which acts on an object towards the centre of its circular path) an object with a velocity (speed and direction) will move in a straight line, unless there are other external forces acting on it.

 

OK, that's enough mental exertion from simple old me...where are the Physicists out there.

 

Bruce

 

 

[facthunter]

Anyhow..

 

You will get a figure from the formula above provided you use the right units which gives a useable answer. I have , over the years specialised in engine balancing and for the situation described 100 percent balance factor should cover the situation, for calculating the load. You would allow an additional margin on this as well, but as I said a "ball park figure". Nev...

 

 

[Yenn]

facthunter. Can you explain to me how the reversal is faster at TDC than at BDC.

 

I am also wondering about the reason for this question as 2 strokes nearly always use manufactured ball or roller bearings and the design of an alternative bearing is going to be a very big job. Is this for a slow revving non aviation engine?

 

 

[facthunter]

Explain.

 

Yes Ian, The point where maximum piston speed is reached is not at the 90 degree or 270 degree position, but at a point where the connecting rod axis is at 90 degrees to the line between the big end and main bearing centres. In every case this is above the "half-way" point and is exaggerated when the connecting rod is made shorter.. The piston assy and the top portion of the connecting rod therefore goes from max velocity in one direction, to the max velocity in the other direction, in less angle of the crankshaft, and therefore in less time, in the area around TDC than the BDC area. The acceleration has to be greater therefore, hence the force is greater.

 

This has a secondary effect insomuch as these accelerations are not equal one does not balance out the other, and this causes the unpleasant vibrations in in-line four cylinder motors that do not have balancer shafts.

 

2-strokes normally do have antifriction bearings, of the roller bearing type, where the crankcase pressure changes are used to scavenge the engines. Where a blower is used to do this then the big ends can be of the normal "bush" design and force fed with oil in the conventional way. Some early motorcycle and outboard and low-powered marine designs had plain bearings ( as do most model aircraft engines ) but they have excessive amounts of oil added to the fuel mix by todays standards Nev..

 

 

[Downunder]

Thanks all for your replies.

 

It will in fact be for an aviation engine(sort of).

 

Lowering of the diameter of the crankpin reduces the rotation speed (Meters/Second) on the bearing.(need this for the material I'm looking at). It revs quite fast as a small 2 stroke would.

 

Normally a needle roller bearing would be used, but the physical size and composition of the conrod becomes an issue.

 

A sintered bronze bearing may have lubrication and heat issues at the oil mix I want to run. More oil = less power.

 

There are many modern materials being developed all the time.

 

If I can find the "force in newtons" that I seek and am able to keep the rotation speed down, then I may be on to something.;)

 

Thanks again,

 

R J Mitchell

 

 

[geoffreywh]

In almost 50 years as a motorcycle mechanic motorcycle I NEVER saw anything but needle roller big ends in two stroke engines

 

 

[facthunter]

Universal.

 

That's true, and I would be skeptical of anything else doing the job. The rollers themselves need practically no lubrication except for the fact that they are thrown together by centrifugal force ( Am I allowed to call it that ?) Where a cage is fitted it's frictional properties help the situation.

 

Downunder, IF you reduce the crankpin dia much ,you will end up with it being the weak point. Are you trying to get the crankcase compression up to a higher level, because it has been found that it doesn't help a lot. Nev...

 

 

[Downunder]

Here is a con-rod with bronze bearings.

 

Oil content required is approx 15 to 20%.

 

Having spent some time looking at advanced plastics, ceramics and alloys, I may be close to trialling a product with better properties than bronze.

 

This would allow a lower oil content. More power and less pollution.

 

There are ways to improve crank-pin strength, to compensate reduced diameter.

 

Heat treatment, material selection, etc

 

Engine manufacturers produce engines to a budget. There is always room for improvement.

 

Regards,

 

R J Mitchell

 

 

 

[facthunter]

Materials.

 

I have been making crankpins for longer than I want to think about it. Even the ones from alpha UK. had a period of failures, at one stage. so if you can't trust the bought one, you can only maintain quality control by doing Or controlling the process yourself. Some pins are 2-piece, the very high tensile centre (or load carrying section) and the carburised or ball bearing quality bush on which the rollers run. There is no low-cost aspect to this. People don't put in roller bearings and built-up crankshafts to save money, they do it because the application demands it.ie Most 2-strokes. Outboards don't have built-up cranks . they have heat treated shafts, ( the metallurgy is a compromise here, being a one-piece shaft) and they have split big end conrods, but they are still roller bearing equipped.

 

HIGH performance 4-strokes have gone away from the rollers in most cases, at the expense of having higher oil volumes and oil coolers to cool the plain bearings and to get a stronger one-piece crankshaft.

 

Drag engines... nitro burning engines have such extreme pressures that a roller bearing will fail virtually on start-up. The load is too high. The rollers crush and destroy themselves.

 

Nev...

 

 

[Downunder]

Thanks for that Nev... lots of good info there....:thumb_up: