Inboard or Outboard clutch benefits

[bmp01]

Which one puts the chain closest to the centreline of the saw ? Does that mean the crank dimensions can be smaller (ie lighter) ? 

 

[Piston Skirt]

2 hours ago, bmp01 said:

Which one puts the chain closest to the centreline of the saw ? Does that mean the crank dimensions can be smaller (ie lighter) ? 

 

Spot on. The PTO can be shortened a bit and even smaller diameter on an outboard clutch version.

Anyway if you don’t want to see a painful smile on experienced chainsaw engineer’s face - avoid asking “inboard or outboard”.

There is no clear answer - both types have their pros & cons and it’s a matter of compromises and quality of execution.

[Paddy1000111]

I'm surprised the PTO side can be shortened on an outboard. Considering that the drive contains the same components I would have thought it would be the same considering you still need all the same bits plus end float room for the chain too. 

[bmp01]

18 hours ago, Paddy1000111 said:

I'm surprised the PTO side can be shortened on an outboard. Considering that the drive contains the same components I would have thought it would be the same considering you still need all the same bits plus end float room for the chain too. 

Yep. Space for chain end float same for both too.

I was thinking about the forces on the crank. Forces ought to be smaller with chain closer to crank bearings. Then I got stuck,  considering the load path from crank to chain. I suspect theres a better case for the outboard clutch, chain forces supported through clutch bearing which is closer to engine centreline. But when the clutch is engaged does the bearing take all the chain force .... or does the clutch ?

 

 

[peatff]

2 hours ago, bmp01 said:

when the clutch is engaged does the bearing take all the chain force .... or does the clutch ?

The sprocket takes it, when the clutch is engaged the bearing is just acting as a spacer.

[bmp01]

1 hour ago, peatff said:

The sprocket takes it, when the clutch is engaged the bearing is just acting as a spacer.

Emmm... dunno.

The 390xp i bought s/h started and ran ok'ish, slightly strange behaviour as clutch engaged. Turned out the clutch bearing had been pinched out of the saw 😲 

On t'other hand the early ms261 fiasco says the clutch bearing/crank has to be up to spec to survive - it clearly works for a living, that says its not just a spacer IMHO.

 

Edited March 11, 2021 by bmp01

[Paddy1000111]

1 hour ago, bmp01 said:

that says its not just a spacer IMHO.

 

I'd say it's only a spacer when the clutch is fully engaged, overload the saw or idle a lot and all that load goes through the bearing. If you're an animal and tighten the chain to F or keep tightening when the chain gets hot and not let it off once you're done with the saw then all that force goes into the bearing, dimples it's running surfaces and then it takes itself out!

[peatff]

1 hour ago, bmp01 said:

The 390xp i bought s/h started and ran ok'ish, slightly strange behaviour as clutch engaged. Turned out the clutch bearing had been pinched out of the saw

So on that note bearings are not really needed ? 261 was a manufacturing problem nothing to do with the bearing, I'm not going to stick IMHO on the end. The bearing works for a living on tickover or when you get the chain wedged and don't let off the throttle.

 

[Piston Skirt]

On 10/03/2021 at 23:14, Paddy1000111 said:

I'm surprised the PTO side can be shortened on an outboard. Considering that the drive contains the same components I would have thought it would be the same considering you still need all the same bits plus end float room for the chain too. 

The bits to fix sprocket assembly are gone on outboard - the clutch shoe holder acts as a master nut whereas on inboard clutch you have to leave around 5mm for a clip groove or a fixing nut thread.

Also, because the chain is substantially closer to the crank bearing there is less levering force from it on the crank PTO part which permits using smaller shaft diameter.

Also moving brake mechanism out of the crankcase allows making it narrower etc.etc...

I know, it sounds “just a few milimeters” but

engineers bother making expensive 2 piece cranks just to eliminate those couple milimeters.

Edited March 12, 2021 by Piston Skirt

[bmp01]

8 hours ago, peatff said:

So on that note bearings are not really needed ? 

Exactly. It did seem to work surprisingly well without brg. Lucky the brake band held the clutch approximately in the right place I suppose. The transition from from idle to chain runnning though .... horrible !

I should say I didn't cut with it like that, took it apart because it didnt feel quite right. 

 

I know the MS261 story. But even the substandard cranks were hardened steel , I still think it's a valid statement to say bearing works hard.

8 hours ago, Paddy1000111 said:

I'd say it's only a spacer when the clutch is fully engaged, overload the saw or idle a lot and all that load goes through the bearing. If you're an animal and tighten the chain to F or keep tightening when the chain gets hot and not let it off once you're done with the saw then all that force goes into the bearing, dimples it's running surfaces and then it takes itself out!

 

Ok, sensible.

But I have an issue with the clutch not running as true as one would hope. They're not that accurately made, carrier is held on with a thread, obviously play between the components for the weights to move. My guess is the bearing clearance is tight enough to do a job of keeping the clutch assembly concentric while clutch is driving the chain. So its carrying a force - I think. How much, dunno.