[Philip Posts: 2436 Status: Offline 12" above the Earth's surface WWW]

Agreed with all points. I seem to recall that a 75x50x3 was needed to keep the deflection within tolerance. And then some accuracy in fabrication too!

Like you say, there’s no wonder people have trouble with these things. Too few Engineers.

Yes going up in size is what cures deflection, thickness of the box not so much. Cant find the figures here, filed somewhere

[Steve Holland Posts: 1582 Status: Offline]

The pto bearing type should be the third number after the displacement. If it's 0 or 7 that would indicate a plain pto, hopefully it's neither of those numbers and has a ball bearing in there.
Photo attached.

[Ian Brooks Posts: 3551 Status: Offline]

Nice. Simple cheap and will keep the deflection under control. Good to see a bit of Engineering going on!

[Ian Brooks Posts: 3551 Status: Offline]

Looks like your not adding any additional support to the briggs drive shaft?  The lower shaft position looks to be a simple hole.
I don't have anything to go on other than my gut feeling, but, surely the crankshaft bearings are not designed for that type of loading. And, if they are, then wouldn't the normal running stresses of the motor account for much of the design loading. The extra loading leading to a short bearing life?

The Briggs PTO bearing is enormous - it’s designed to take belt loads in addition to the engine loads.

[Al Posts: 880 Status: Offline WWW]

Jon,


Private message me with your email, as I have the 37efi manual on pdf.  I can't remember where I downloaded it from, but hopefully the file will be small enough to email  (4.86 mb)

[Gaz Posts: 2348 Status: Offline Cruising around Cornwall WWW]

OK, I was just a little concerned it may be a lot for the bearings, if your happy, its your craft after all lol.

EFI: Not done anything with a reader or tinytac as yet. I'll get the colour codes for you. I also have a full complement of manuals for it too (with fault finder/ codes etc). everything is at the build site so give me a few days and i'll dig them out.

[joncurtis Posts: 2068 Status: Offline]

hi gaz
yeh did think of the suitability of the briggs bearing. im not making the shaft any longer, im just sticking the pulley directly to the shaft. Id be surprised if the briggs bottom end is not capable of taking the load from a pulley and a belt. but yeh it might not be ok. only time will tell. im more concerned the fan frame with rip the back plate off the engine

people do drive belts directly off the briggs i have seen this done a few times. mainly in flying fish which are about as reliable as a beko washing machine. but never seen the bottom ends fail. im not over reving it

did you say you have an EFI, do you have one of the tinytac RMP/Fault code readers? do you have a wiring diagram for the engine.
when it gets a bit warmer and heath permitting im building a shed and will get the engine out and have a play with it

[Gaz Posts: 2348 Status: Offline Cruising around Cornwall WWW]

Looks like your not adding any additional support to the briggs drive shaft?  The lower shaft position looks to be a simple hole.
I don't have anything to go on other than my gut feeling, but, surely the crankshaft bearings are not designed for that type of loading. And, if they are, then wouldn't the normal running stresses of the motor account for much of the design loading. The extra loading leading to a short bearing life?

[joncurtis Posts: 2068 Status: Offline]

got the deflection down to 0.14mm, thats probably good enough/within the tolerance of the analyis stiff enough.

average stress is well within the material properties.

this is what i have been upto, if anyone is interested. I have access to software through my work, and found a briggs block model on the internet, its not 100% acurate, but good enough.

might tie the frame into the engine top, to not load the engine back plate, so got more to do.

the frame model is one solid part, i normally model the welds to see the stress within them, the top shaft is short and cut to allow the downward force to be applied, assumed that the majority of the load would be on the first bearing and i already know the shaft is stronge enough.

i spent ages fannying around putting plates inside the assembly and all sorts of webs, the strongest solution is just weld two bits of box together, probably the best from a heat distortion too. the frame weights just under 6kg.

[Ian Brooks Posts: 3551 Status: Offline]

Agreed with all points. I seem to recall that a 75x50x3 was needed to keep the deflection within tolerance. And then some accuracy in fabrication too!

Like you say, there’s no wonder people have trouble with these things. Too few Engineers.

[joncurtis Posts: 2068 Status: Offline]

mmmmh done some analysis and not convinced 50mm box section is stiff enough.

Im sure you mentioned this before ian a few/several years ago.  on 507.5mm centres i get 0.5mm of deflection on 50x3mm wall mild steel box. which would explain why htd belts wonder about on most craft!

[Ian Brooks Posts: 3551 Status: Offline]

Yes, the force is the static load on the bearings that you would use to design them.

The adjustment is needed to tension the belt once it’s on. You’ll struggle to get it on otherwise.  Could be done with shims.

If you’re having no adjustment, do check the tolerances on angles, centres etc - they are tight! 0.25 deg if I recall correctly. And they need to be held when the load comes on, so don’t forget the stiffness.

Ian

[joncurtis Posts: 2068 Status: Offline]

hi

in the HTD drive belt calculator on the site, am i correct in assuming the shaft static load is the load put on the shaft by the belt force, so i could assume this would be the force pulling down the driven shaft (fan shaft)?

also whats the adjustment needed for tensioning bit? thats 2mm, im going to have fixed centres so do i need to worry about this?

if anyone is interested,

HTD 8m

44tooth bottom pulley62 tooth top1440 belt 507.5 centres is what it tells me, i have checked this with another calculator as Ricky, told me to use 527.7 centres, so i assume his was a typo.

and if your still reading, im going to use a 7 of 7 920mm dia fan, with this 37efi briggs. which was what was recommended by ricky, and sounded sensible, so went with that.

only a little craft 10ft long or so, light weight hull, similar size to an osprey really