Author Topic: Big end bolts  (Read 5162 times)

Offline Roger B

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Big end bolts
« on: November 20, 2015, 08:00:10 PM »
I am starting to sketch up my next engine. It's a two stroke horizontal diesel, loosely based on the Field Marshall tractor engine. Bore 25mm stroke 40mm (~20cc).
As it's planned to be a true diesel I am looking at at least 20-1 compression and as a two stroke I want to keep the free volume in the crankcase as small a possible.

On my previous engines I have used M3 big end bolts with an 8mm crankpin and M5 big end bolts with a 12mm crankpin. These seem large in proportion to those on full size engines. This engine will have a 12 or 14mm crankpin and to keep the crankcase compact I am thinking of using M3 bolts. I could do some stress calculations (as it's a two stroke the con rod is in theory always in compression) but I always like real world advice if it's available  :)
Best regards

Roger

Offline Brendon M

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Re: Big end bolts
« Reply #1 on: November 21, 2015, 04:52:37 AM »
Hello Roger

I would start looking at similar sized glow engines and look at the bolts for the big end bearing, or if you can't find any models using split bearings, have a look at the bolts holding the head on. (Someone correct me if I am wrong, but they should be in constant tension).

Otherwise, I know you have asked for real world data, but if it helps, a 3mm bolt could theoretically hold a bit under 3 kilonewtons, which is about 300kg at sea level! (Mind you, I am not sure what the fatigue limit of these bolts are).

As you say, theoretically the bigend should always be experiencing compression, but I would suspect there could be tension just after TDC as the piston begins to move down for the power stroke (I understand that injection timing actually starts a few degrees before TDC but I can't say how long between injection start to actual pressure starting to rise and act on the piston)

There is also the scenario when you back off the throttle. Maybe a good thought experiment is to consider how much force the piston will generate at X RPM with no fuel, and see if the forces generated a half of what the bolts are capable of holding?

edit: sorry, that got a bit long!
(This signature intentionally left blank)

Offline Jo

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Re: Big end bolts
« Reply #2 on: November 21, 2015, 08:25:05 AM »
The Kiwi is a similar size and used 5Ba big end bolts.... Seal Major also 5BA. The Wall Wizard uses 5-40 UNC but it is a squarer engine.

Most of the engine thrust is pushing rather than pulling the rod so so long as you use off the shelf high tensile bolts I can't see that you will  have a problem.

Jo
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Offline bp

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Re: Big end bolts
« Reply #3 on: November 21, 2015, 09:57:22 AM »
I know it's a bit like comparing a goldfish with a whale shark, but.....
On the Norton Commando engine, a 750cc twin cylinder four stroke, producing about 50/55 b.h.p., hardly enough to push a scooter along these days, the big end bolts and nuts were just about the most highly specified part on the whole motorcycle.  Material, dimensions, shape, surface finish, heat treatment, EVERYTHING was nailed down.
Bearing in mind my first sentence, this would lead me to think that these bits are considered to be pretty important and probably best not to be trivialised.  So I would be searching the catalogues of people like Unbrako and Holo-Krome.  At least their stuff isn't made of shiny plasticene, like most of the nuts and bolts on the market at the moment.
cheers
Bill

Offline Allen Smithee

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Re: Big end bolts
« Reply #4 on: November 21, 2015, 10:44:03 AM »
I would be careful with that assumption about the rod always being in compression. It's a long time since I did any of these sums, but from what I remember as soon as the revs get to non-trivial values in most engine configurations the stresses due to reciprocating motion are always much larger than the stresses due to compression and combustion, so the rod will be under tension for at least 25-30% of the cycle.

All the rod failures I've ever seen have been tensile or fatigue failures, and you can't get fatigue without a cyclic tensile stress.

AS
Quidquid latine dictum sit altum sonatur

Online sco

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Re: Big end bolts
« Reply #5 on: November 21, 2015, 04:36:53 PM »
Yep the inertia of the piston means that approaching TDC the rod will be in tension and the higher the RPM the higher the force will be and can easily exceed the compressive force due to combustion pressure.

If you know the RPM and the weight of the piston and conrod you can calculate the peak tensile force, similarly you can calculate the preload in the M3 screws and you would want to avoid the joint gapping (the rod tensile force exceeding the preload).

Simon.
Ars longa, vita brevis.

Offline Roger B

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Re: Big end bolts
« Reply #6 on: November 21, 2015, 08:06:46 PM »
Thank you all for the thoughts and suggestions :ThumbsUp:  :ThumbsUp:

If this engine ever runs I doubt if it will do more than 5-6000 rpm. I may use a CI piston to keep a close fit in the CI cylinder at all temps so I will need to rough this out to estimate the weight. As the Kiwi/Kitti use 5BA which is pretty close to M3 I'm at least in an area of reality. I would like to keep the big end small enough to allow it to pass through the cylinder to ease some other parts of the design.
Best regards

Roger

Offline Jasonb

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Re: Big end bolts
« Reply #7 on: November 21, 2015, 08:33:34 PM »
Roger do you not have a CAD package that can work out the weight of a part you have drawn?

Offline Roger B

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Re: Big end bolts
« Reply #8 on: November 21, 2015, 08:40:40 PM »
I do, but I have to draw it first  ;) This engine is still mostly in my head with a few pencil sketches of potential problem areas.
Best regards

Roger

Offline Roger B

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Re: Big end bolts
« Reply #9 on: November 23, 2015, 12:16:02 PM »
Time for some calculations  :headscratch:

Acceleration at TDC = 0.5*(stroke)*(w^2)*(1 + 0.5*stroke/rod_length) where w is the angular velocity  = π *rpm/30.

For a 40mm stroke and an 80mm rod at 6000rpm the max. acceleration is 9860 m/s2.

The material volume for the piston is around 10 cm3 and the connecting rod is similar. I will calculate  based on an aluminium piston and rod as well as a cast iron piston with an aluminium rod.

For both aluminium the mass is 27 + 27 = 54g

For CI and aluminium the mass is 72 + 27 = 99g

Using F=ma the force will be for an Al piston 532N and for a CI piston 976N

The UTS for an 8.8 M3 thread is around 4000N and for a 12.9 around 6000N based on a load carrying area of 5mm2 . So even in the case of the CI piston I will have a fairly good factor of safety without compression  :)

I am looking at a compression ratio of 20-1 so taking atmospheric pressure at 0.1N/mm2 the pressure will be (roughly) 2N/mm2.  The piston area is 490mm2 so the compression load will be approx. 980N or about the same as the acceleration force for the CI piston  ::)

Please don’t base any of your own work on these calculations without checking them. I may be a factor of 10 out due to typing errors  :facepalm: (this I why I was asking for real life figures at the start).
Best regards

Roger

Offline Allen Smithee

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Re: Big end bolts
« Reply #10 on: November 23, 2015, 06:26:09 PM »
I'm assuming that 20:1 is a geometric CR, so for an un-piped and un-supercharged 2-stroke the actual pressure will be a lot lower...

AS
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Offline Roger B

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Re: Big end bolts
« Reply #11 on: November 23, 2015, 07:02:22 PM »
The geometric ratio is aimed at 25-1 although the bumping clearance will have a significant effect. With the heat losses of a small engine I am hoping a real(ish) 20-1 will give enough heat for ignition. Proper sized direct injection engines will cold start at around 14-1.
Best regards

Roger

Offline Admiral_dk

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Re: Big end bolts
« Reply #12 on: November 23, 2015, 09:21:46 PM »
Very interesting project  :cartwheel:

Quote
Proper sized direct injection engines will cold start at around 14-1.

I guess that this might be true with engines over a certain number of cc's per cylinder - a former boss of mine, told me that, when he was a lieutenant in the army, they often push started the big trucks without any problems (just a few men), but even a lot of men couldn't do the same with most small car engines if they where somewhat down on compression (lower that the 20-21:1).

Offline Roger B

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Re: Big end bolts
« Reply #13 on: November 24, 2015, 07:26:47 AM »
By 'proper sized' I am thinking of a bore more than 100mm. A lot of car engines use/used indirect injection systems for smoother running and higher speeds. These generally required heater plugs for starting so unless you could turn the heater plugs on when pushing they won't start.
Best regards

Roger

Offline johanvanzanten

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Re: Big end bolts
« Reply #14 on: December 07, 2015, 05:53:49 PM »
Hi Roger,
I made à diesel engine with 25mm bore and 40mm stroke. IT is à slow 250 rpm reving crosshead engine.
The bigend diameter is 12 mm and I used M5 bigend bolts. Succes with your engine.
Regards,  Johan.

 

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