Saw Modders...take a look at this!!

[Nitroman]

You may also consider having the piston coated on the skirts to reduce friction. If you are worried about slap, you can have the diameter increased with the coating to reduce the wall-piston clearance. This is a popular mod for hi-perf snowmachine engines, and especially the larger bore engines.

Much information here:

http://www.swaintech.com/

 

[KennyW in CT]

Any thoughts about rebalancing the crank to allow for the drop in reciprocating mass? Will the out of balance vibration cause failure?

 

[sILlogger]

Any thoughts about rebalancing the crank to allow for the drop in reciprocating mass? Will the out of balance vibration cause failure?

this is my take on it..someone much more educated and experienced can elaborate more...but..

a chainsaw being a single cylinder with the piston only moving in a linear pattern(up and down) that lightening the piston will not affect balance because it is part of the reciprocating mass that does very little to no side to side movement and is largely contained to straight up and down. and the crank itself is also balanced laterally and held in the bearing cradle (theoretically should have very little runout)

and with only one cylinder that is only one piston that is putting a load on the crank..as opposed to multiple pistons that would be in different locations in their respected cylinders during the rotation

I know what im trying to say..i just don't know how to say it

 

[klickitatsacket]

single engine cylinders are balanced for a particular RPM range. When you lighten a reciprocating mass you actually will help vibration.

 

[sILlogger]

ahhh ha...dean is here!! can i get your input on the piston...ups & downs? tips? your help insight would be greatly appreciated

 

[klickitatsacket]

They let me out of jail. :monkey:
This is only my opinion. There will be those that disagree with me.
Teh bottom web is for side presure support for the skirt and has little to do with the wrist pin being supported. There are bike pistons that do not have any support below the pin as there is no skirt below the pin. What hapens is on a longer skirt piston you get side preasure down low as the crank rolls over at TDC and BDC. Think of it as a teeder todder with the wrist pin being the fulcrum and as the crank rolls over and piston wants to tip back against the roll over direction the opposite corner presses against the wall of the cylinder. When you get piston slap as the piston wears down you will more preasure bing applied to a smaller area of the skirt instead of spreading it out over a larger area. This will cause skirts to break or crack.

Now if the supports where not there then the skirts would flex and break right away. Just remember that the supports are for side preasure any how and not for wrist pin support.

Let the thrashing begin. :greenchainsaw:

 

[sILlogger]

They let me out of jail. :monkey:
This is only my opinion. There will be those that disagree with me.
Teh bottom web is for side presure support for the skirt and has little to do with the wrist pin being supported. There are bike pistons that do not have any support below the pin as there is no skirt below the pin. What hapens is on a longer skirt piston you get side preasure down low as the crank rolls over at TDC and BDC. Think of it as a teeder todder with the wrist pin being the fulcrum and as the crank rolls over and piston wants to tip back against the roll over direction the opposite corner presses against the wall of the cylinder. When you get piston slap as the piston wears down you will more preasure bing applied to a smaller area of the skirt instead of spreading it out over a larger area. This will cause skirts to break or crack.

Now if the supports where not there then the skirts would flex and break right away. Just remember that the supports are for side preasure any how and not for wrist pin support.

Let the thrashing begin. :greenchainsaw:

i follow you....so how do you think that will correlate to my piston? u think it will work?

 

[spacemule]

There can be a good 1000 lb of force pulling up on the pin boss in some cases, and easily 3000 lb pushing down,

I admit this is completely conjectural speculation on my part, but aren't the forces on the pin bosses all upward? When the piston is rising, compression is pushing downward on the piston which in turn pushes upward on the pin bosses in relation to the piston. When the piston is descending, ignition is pushing downward on the piston and in turn upward on the pin bosses.

 

[Crofter]

I admit this is completely conjectural speculation on my part, but aren't the forces on the pin bosses all upward? When the piston is rising, compression is pushing downward on the piston which in turn pushes upward on the pin bosses in relation to the piston. When the piston is descending, ignition is pushing downward on the piston and in turn upward on the pin bosses.

The decelleration G forces approaching TDC are greater at high rpm than the compression forces so the net force puts the connecting rod in tension. This might not occur in a stock saw rpm. Forces under combustion can easily exceed 1000 psi so there is higher compression force on the piston. Because of connecting rod angularity a vector force is exerted on the skirts and this portion is transmitted to the wrist pin through the lateral supports. The forces are not simply up or down.

 

[PES+]

The decelleration G forces approaching TDC are greater at high rpm than the compression forces so the net force puts the connecting rod in tension. This might not occur in a stock saw rpm. Forces under combustion can easily exceed 1000 psi so there is higher compression force on the piston. Because of connecting rod angularity a vector force is exerted on the skirts and this portion is transmitted to the wrist pin through the lateral supports. The forces are not simply up or down.

Only in a non balanced engine or an offset cylinder bore configuration

 

[timberwolf]

No, but there are cases where there is no tension force on the boss too. As the piston is coming up towards TDC WOT, there is about 4,000 g of acceleration required to slow the piston to a stop at TDC, put that together with a 1/4 lb piston. When the saw is firing and a full charge being drawn in yes, early combustion and compression does off set acceleration totally or at least mostly. However the problem happens when the saw is at full throttle and the throttle plate closed, what the saw is compressing each stroke drops way down and there is nearly zero combustion, at least no combustion that makes much pressure. I did up some calcs on this before and posted some graphs showing the loading on the rod through one revolution, this showed what part of the load was from compression and combustion and what part from piston acceleration.

What Dean is saying is in line with what I have read on it. Yes the piston mass is balanced to the RPM and bore and stroke of the saw, working some of the existing pro saw designs backwards showed that in stock form the loads on the bearings were carefully engineered to be in balance with the compression force andoff set piston acceleration nicely. But start to mod a saw and bearings could certainly be pounded on by piston acceleration. But there is no RPM relationship between crank mass and piston mass acceleration between piston and crank, the faster the crank goes the faster the piston goes, so that symple geometric balance is independent of RPM changes. But the balance between bore/compression and piston mass is going to be linked to RPM so it can only be right at one speed .

Single cylinder balance is just a compromise and really only takes some of the up and down viberations and turns them into side to side viberations, if the piston is light for the crank the up and down viberations would be less and side to side the same, if the crank was lightened to match the piston weight reduction both up and down and side to side viberations could be reduced slightly. By what I have read, removing 1/2 the reduction in piston weight from the crank bells is a good start.

But for a work saw it's not going to make a hill of beans difference.

 

[PES+]

I thought we were talking theoretical and in ultimate possible.

Over revving a lot of engines is asking for connecting rod separation for different reasons in different configurations but it is possible to nearly eliminate extension force on the rods by careful design and forethought.

It is not practical to do so on a large scale production basis though at the current competitive price points

 

[Crofter]

I thought we were talking theoretical and in ultimate possible.

Over revving a lot of engines is asking for connecting rod separation for different reasons in different configurations but it is possible to nearly eliminate extension force on the rods by careful design and forethought.

It is not practical to do so on a large scale production basis though at the current competitive price points

Pes+ there are a whole lot of people here modding saws though and undoing all that careful design and forethought, Lol!

A constant direction load is much less contributing to metal fatigue than stress reversal. Unloading and reloading bearings during direction reversals also leads to bearing element skidding and fatigue in bearing cages.

 

[PES+]

Pes+ there are a whole lot of people here modding saws though and undoing all that careful design and forethought, Lol!

A constant direction load is much less contributing to metal fatigue than stress reversal. Unloading and reloading bearings during direction reversals also leads to bearing element skidding and fatigue in bearing cages.

Which is why one always attempts to eliminate the stress reversal

There are many ways to test and verify when you are close....but none involve caged needle or roller bearings which are quite unsatisfactory for really high RPM

 

[timberwolf]

Here is what I was working on before on the acceleration forces. Just a slick little spread sheet where I can punch in the bore, stroke, rod length compression ratio, RPM and how much everything weighs and come up with a little insight on the loading on rods and bearings.

http://www.arboristsite.com/showthread.php?t=53484&highlight=rod+tension

 

[Lakeside53]

Which is why one always attempts to eliminate the stress reversal

There are many ways to test and verify when you are close....but none involve caged needle or roller bearings which are quite unsatisfactory for really high RPM

what's "really high" (in the saw world)?

 

[Simonizer]

what's "really high" (in the saw world)?

4 joints and 6 shots of Jack Daniels?

 

[Lakeside53]

4 joints and 6 shots of Jack Daniels?

As long as that's in 2 hours, I'd (well... at one time) agree.

:greenchainsaw: :greenchainsaw: :greenchainsaw: :greenchainsaw: :greenchainsaw: :greenchainsaw:

 

[RiverRat2]

4 joints and 6 shots of Jack Daniels?

LOL!!!!!!! Extemely High!!!!!

As long as that's in 2 hours, I'd (well... at one time) agree.

:greenchainsaw: :greenchainsaw: :greenchainsaw: :greenchainsaw: :greenchainsaw: :greenchainsaw:

In two Hours???? you would have to put me in a wheel barrow and roll me about!!!! LOL !!!!!! And dump me on the sand bar!!!! under a shade tree Please!!!!!! (eerrr, well at one time!!!!)

 

[Simonizer]

Sorry, didn't mean to de-rail the thread. Technical things kinda confuse me. (I don't really know stuff about things.)