Which loctite to use

[MarekH]

There are a few potential contradictions in that article.

"All this mayhem..." essentially says do it up to the recommended correct torque and it'll work because it won't cyclicly stretching and compressing all of the time.

But it then also tells us we can throw away our torque wrenches because it is all about stretch.

Meanwhile it also tells us that at BDC the bolt "returns to original length" - well if that were so, then it is only a tiny bit of friction holding the nut from turning on the bolt.

Don't you just love the interweb?

Logic says that the Loctite will only help to make sure that the nut doesn't move if it hasn't been done up tight enough in the first place. Put another way it helps prevent the consequences of a failure but it ought not to be instrumental in achieving that correct result in the first place.

kind regards
Marek

[Hugo]

Well, that's a clever point, Christopher. So many things turn out to be counter-intuitive when you start thinking about them. But would not the maximum force occur when the piston suddenly changes direction, rather than when it is slowing down on the way up? Or is that a factor of con-rod length and stroke? There is a certain degree of friction acting as a brake on the piston on its way up, which is pretty much absent when it hits TDC. As I read the article, the maximum tensile force occurs when the fixings are set to the correct torque. Nothing that happens when the engine is running will approach that level (unless you over-rev it of course). If you think of the bolt as a spring, if it is a strong enough spring, it won't be stretched any further by the actions of the piston in normal operating conditions.
As for the 'maximum compressive load', that won't affect the big-end bolts, will it? It will either bend a con-rod or break the crank if that load gets too much.

[Hugo]

There are a few potential contradictions in that article.

"All this mayhem..." essentially says do it up to the recommended correct torque and it'll work because it won't cyclicly stretching and compressing all of the time.

But it then also tells us we can throw away our torque wrenches because it is all about stretch.
Marek

I don't see a contradiction - the writer is saying that you need to stretch the bolt initially by more than it will encounter in normal running, so it won't notice the additional stretching forces.
Bear in mind that this is from an American forum. With all due respect to our Colonial cousins, I find Americans are very sloppy when it comes to the written word (or the spoken word for that matter!). At least that is my perspective when I'm in England. When I am in America (which is about 1/3 of the time) I find that if I try to write correct English (which I do try to do, not always successfully) it comes across as very pedantic. Modern America, of course, is a melting pot of different languages all melded together into a form of English. Surnames are a case in point - in England you can usually guess a person's name if you can't read all of it, because the name will be one you are familiar with. In America you find a bewildering array of names that seem to bear no relation to any known language. This is often because they were originally spelled phonetically in a different language.
Drifting off topic again

[christopher storey]

Hugo - the tensile force on the bolts is at a maximum when the g force is at a maximum. If you think about it just in rough terms, at say 4800 rpm the average piston speed is roughly speaking 8 inches per second X 80 = approx 54 feet per second and therefore the peak speed approximates to twice that = 108fps . The piston reaches that speed and stops from it 160 times per second ( once on the way down and once on the way up in each rev) and thus ( and this is a gross approximation because much depends on the variables of con rod length etc and ratio of stroke thereto ) the guestimate of peak g force is 108 X 160 /32.2 g = 536 g . If the piston weighs say 1 lb or 0.5 kg, the tensile load on the bolts is of the order of 1/4 tonne i.e. its weight X 536g. Contrast this with the point of reversal at TDC/BDC where the linear motion , and therefore both the speed and the concomitant rate of acceleration of the piston is negligible

[Hugo]

I'm almost with you. I was going to ask where the 32.2 grammes came from till I figured out it was the 'other' G!
Actually I think it's even more complicated than that; you also have the weight of the con-rod to take into count. But where it gets really interesting - and well beyond my knowledge of mathematics - is trying to figure out where in the bore the piston reaches peak speed. All I can say for sure is that it's going to be more than half way down. Or less than half way up if you prefer. If you had an infintely long con-rod it would be half way up (or down), but because the con-rod is swing in an arc of the same orientation as the crankpin, there is going to be more 'dwell' at the bottom. Conversely it's going to do a much quicker reversal at the top of the stroke, so the reversal of motion is going to be much more violent than it is at the bottom of the stroke. Intuitively I feel that that is the point where the greatest G force should occur, but you seem to be saying that is not so.
How about bungee jumpers? Where do they experience the greatest G force? Is it when they're braking as the rope approaches its maximum 'stretch', or is it when they dip their hair in the river below before springing back up again?

[christopher storey]

Hugo : we'll bore everyone to death, but a final answer is that the greatest linear speed is reached at that point of the motion where the axes of conrod and crankpin are at right angles to one another. This is why acceleration either positive or negative is greatest at that point , and why acceleration is least at TDC/BDC because at those points each degree of crank rotation ( which for these purposes can be taken as being at constant speed ) produces least linear motion. If graphically represented, the piston speed as against crankshaft position is a sinusoidal curve

[malcolm]

HELP!!

[mgcjag]

Hi Guys.....yes this has now moved way off topic "Which loctite to us"......PLEASE continue the discussion if you want under a seperate heading.....anymore on Loctite then keep going...thanks... Steve

[Hugo]

You never stop learning, do you. Not you - me! I had always believed that the highest piston speed was attained when the crank-pin was at 90 degrees, but as you point out it is actually when it is slightly uphill from that point.
That is where maximum piston speed is attained, but surely it can't also be where maximum acceleration occurs, since it's already travelling flat out! Maximum acceleration (or deceleration) has to be a little bit either side of that point, although I'm not sure exactly where!
But I think you're mistaken when you describe the motion of the piston as a 'sinusoidal curve'. (I had to look that up - if you had said 'sine wave' I would have known what you meant!). The 'dwell' at the bottom of the stroke is a lot longer than it is at the top. At the bottom of the stroke the piston intially travels quite slowly, the accelerates to a maximum somewhere........ Now, this is where it gets even more complex; the maximum piston speed occurs when the crankpin is just over half way up - when the axes of crankpin and con-rod are at 90 degrees, as you pointed out. I had always believed that the PISTON (as opposed to the crankpin) would be LESS than half-way up at that point. But now I'm wondering if that point (where the axes are at 90 degrees) is actually the oint at which the iston is half way up? Bored? You will be!

The problem I have with your explanation is that it doesn't seem to take into account the piston's reversal of motion at the top of the stroke - or have I missed that part? The forces are going to be much greater at the top of the stroke than at the bottom because of the relative arcs of the crank-pin and con-rod; the question is, do these forces exceed the deceleration forces imposed on the piston as it brakes for the turn at the top? Intuitively I would have said no, but you seem to say that is wrong? If you imagine a racing car braking hard at the end of a straight before a hair-pin bend (which is effectively what the piston is doing), where is the driver going to lose control? At the apex, surely?

[Series1 Stu]

Christopher is absolutely correct about the point of maximum g being at the points where the crank pin and con rod axes are perpendicular to each other but that has very little to do with the stretch in the bolts.

When tightening bolts, as had already been stated, you are setting up a tensile load of a magnitude sufficient to hold the assembly together throughout the range of conditions it needs to endure. Taking account of loads generated by rotation, reciprocation, heat, shock and any other forces that may be at work. The size and number of the bolts is determined by that total, worst case, load. The bolt is usually tightened to a tensile load approaching, but not exceeding, it's rated proof stress. We have no sensible way of determining what stress we actually have in each bolt whilst tightening it so we use either a torque figure or an amount of stretch as a proxy for tensile stress. These figures are calculated from the rated mechanical properties of the bolts. Stretch is the more difficult of the 2 to measure because you can't often access the bolt to measure it but the benefit is that it doesn't matter whether the bolt was assembled dry or with lubrication, the tensile stress will be the same. Calculating the torque figure is as reliable as the amount of stretch but you need to factor in whether the assembly is dry or not and then specify it when publishing the torque figure. As a rule of thumb, you would reduce the "dry" tightening torque by about 20% if assembling "wet".

Once you've set up the tensile load in the bolt, the loads it sees in service will never exceed the load set up in assembly so it should never fail unless there is a massive change in loads, such as a seizure.

There are no compressive loads on the bolt as it will never see a situation where there is no stretch on it so suggesting that the bolt shortens in service is fallacious. If it did then the nut would certainly loosen and the whole assembly would end up punching through the cylinder block.

A lot of people don't realise that untightening torque is significantly lower than tightening torque. All things being equal.

Regards

PS don't use Loctite on big end bolts.

[Hugo]

Interesting - a couple of points;

1) In your first sentence, don't you mean maximum speed rather than max g? And the deceleration going uphill from that point has to be different from the deceleration going downhill, because of the arc of the crankpin, which either co-incides with or fights against the arc of the con-rod, depending which way the piston is going.

2) Why would anybody torque anything dry? Seems like bad practice to me? As an aside, since they stopped using L/H thread wheel-nuts on the left wheels of coaches and buses, now have a thing called 'wheel loss syndrome', which is self-explanatory I hope. It's always the near-side rear wheels that fall off. In fact it is not a 'syndrome' at all - it happens because people fit the wheel-nuts dry, and also because they don't clean the mating faces.

3) You say the 'loosening' torque is less than the 'tightening' torque. Why would that be?

[Series1 Stu]

Hi Hugo

Briefly,

1. No, for the reasons Christopher quoted.

2. I've no idea what that particular problem is but can only suggest that they've used the wet torque figure for a dry assembly when they should have used a figure 25% or so higher.

3. Because, on tightening, you have to overcome a significant amount of friction along with raising the tensile force in the bolt. Try it for yourself. Tighten something to, say, 25Nm then set your torque wrench to 10Nm and try undoing it. Increase your torque in 2Nm increments and see at what level it comes undone.

Regards

[Hugo]

1) Hmmm. Maybe I'm missing something, but I can't see how there is any appreciable force acting on the big-end bolts when the crankpin is pointing sideways?

2) Government vehicle inspectors are fanatical to the point of hysteria when it comes to regularly torquing commercial vehicle wheels to prevent them falling off. They completely miss the point that the way to stop wheels falling off is to fit them properly - i.e. clean all mating faces and grease the threads (I use copper-grease). I never torque my wheel nuts - I just do it by feel - and they never come loose.

3) I'll try that - thanks ;) I do know that when re-tightening head bolts, for example, you have to back the nuts off before re-torquing them, otherwise they won't move because of 'stiction'.

[Series1 Stu]

Sorry Hugo.

Regarding point 1 I never meant to imply there was any appreciable load on the bolts in the circumstance. You asked the question about the point of maximum acceleration.

When it comes to wheel bolts, each to their own. I would always recommend using a torque wrench. I've seen a lot of wheel studs that have been "necked" due to over tightening.

Regards

[Hugo]

Gotcha. I was think 'acceleration' as it related to the tensile stresses on the bolts. Wheel nuts, sump plugs and spark plugs - these are the things that regularly get over-tightened by owners doing their own maintenance.

[driver]

In the jaguar service manual there is no mention of loctight or any other assistant, Just the correct torque settings.

[chrisfell]

All this mention of reliance on torque settings, and therefore the use of torque wrenches, reminds me, I must remember to check that I slackened off the torque wrench after I'd used it to check (re-check) the wheel nuts on SWMBO's S2000 after I had the wheels off yesterday.