Greasing all bolts
Posted April 26, 2007 - 01:05 PM
For the past couple years I have actually been using blue loctite - and the past year or so I have really liked the blue loctite that is solid and comes in the twist up tubes like Chapstick.
A few years back I actually tried greasing the threads of some bolts - notably the motor mount bolts on my 99 YZ450F. Unfortunately, before I was close to the recommended torque, the threads in the frame totally stripped out, and I had to end up using Time-serts in the frame.
So, I'm concerned about repeating this horror on my new bike (07 450). Also a concern is the tendancy for 4 strokes to vibrate and loosen bolts, which I think would be even worse with grease instead of loctite.
What do you guys use - grease or loctite?
Posted April 26, 2007 - 01:16 PM
Posted April 26, 2007 - 01:39 PM
When you use a torque wrench, you are measuring the twisting effort required to turn the bolt, but you are doing so to determine how much clamping tension is being applied to the threads and the bolt shank. Lubricating the threads makes the bolt easier to turn, which means you will turn it farther to get the same resistance to turning, and thus, you will have increased the clamping tension on the bolt by as much as 25% by the time you hit the same torque value.
Angle torquing is a way around this. Take the dry bolt and torque to a 1/4-1/3 of its published dry torque spec. Then, mark the bolt and the part, and torque to its full value. Note how far it has turned.
Now, with the lubed or Loc-Tited bolt, torque to the same 1/4-1/3 torque you used earlier, then tighten to the same angle of rotation as when it was dry.
Posted April 26, 2007 - 03:40 PM
Posted April 26, 2007 - 05:37 PM
If you want to put in anti-seize to help with disassembly, it can be a hit-or-miss proposition getting the right torque. What I prefer to do if I want to make sure it won't vibrate loose is put blue loctite on the couple threads which will just enter what you are bolting to so it will also keep moisture from getting in and rusting the bolt while not affecting the friction between the threads greatly.
Another thing to be careful of is putting lubricant into the threads of a blind hole or on the end of a bolt. If you do this and the bolt comes close to the bottom of the hole, it is possible to create a great amount of hydraulic force at the end of the bolt and break whatever the bolt has threaded into, especially with thicker lubricants like grease, because it cannot escape back between the threads to release the pressure. So if you choose use lubricant only put it on the male threads and start it at least one thread from the end of the bolt.
Posted April 26, 2007 - 08:33 PM
rbinstoon's advice is good, especially that concerning blind holes. But if you're going to lube a bolt that has a torque spec for dry threads, the most reliable way to avoid trouble is the angle method I laid out. You'll see this method specified for the head bolts on the 450.
Posted April 26, 2007 - 09:20 PM
From my investigation of it (I'm a millwright, not just bored), about 85-90% of the torque applied to a fastener goes to overcoming friction, so only 10-15% is put towards stretching the bolt to get proper tension. With so little of the force applied to tensioning, that is why a little lube can make such a difference in bolt torquing(as can several other variables).
Even with clean, dry threads I usually go to about 75% of rated torque to start then go up in a couple steps after that and using feel while doing it to make sure it is still tightening, not stripping. If I can't get them completely clean I go to around 50% and use feel after that.
Anyway, what gray said was right, snugging a bolt to a specified point and then turning it a set amount is a much more accurate method of getting proper tension on a bolt if it can be done.
Posted April 26, 2007 - 09:30 PM
Posted April 27, 2007 - 05:55 AM
I tighten all non-critical items that are removed frequently (seat, radiator shrouds, number plates, etc.) with a hand held nut driver. These are the bolts that are most subject to stripping, but it is hard to strip them when you hand tighten them.
Posted April 27, 2007 - 05:59 AM
And you're right, you'd be amazed if you check it with a torque wrench at how much less toque you need to turn it farther.
Posted April 27, 2007 - 07:56 AM
Posted April 27, 2007 - 09:49 AM
The cool thing about Loc-Tite is that it has enough residual effect to be reusable. It won't work as well when reused as it will on a fresh application, but it clearly works better than nothing.
Posted April 27, 2007 - 10:23 AM
Posted April 27, 2007 - 03:48 PM
When you say grease I assume you mean assembly lube?
Nope,just some red grease out of a grease gun..I just wipe a little on the threads and screw them in..Ive been wrenching on bike for 5 years now..Im a machinist by trade,so I understand threads..this just prevents corrosion and makes disassemby easier..I also rarely use a torque wrench..maybe on cam caps,triple clamps and cylinder or head bolts..I have a good feel for aluminum..I never strip out threads.
When I first started wrenching on my yammie all the factory tightened bolts had corrosion in the holes and bolts were a bear to get out..If I dont like the way threads look or the head of the bolt it gets replaced with socket head cap screws.
I dont use any locktite either..
Posted April 27, 2007 - 08:07 PM
It mainly comes down to bolt preload/fastener tension. When you're applying torque to a fastener, you are essentially stretching or preloading the fastener. This is done to prevent the fastener from relaxing too much and the nut from coming loose - or even worse, falling off. The big question is “how much preload do I need?”
The basic rule we use is that for structural applications, the fastener preload should be 67% of the yield strength. This value was chosen because a fastener (i.e. bolt) has a proof load value for each grade and diameter. The proof load is usually 90 percent of the yield strength. The proof load guarantees that the fastener will not permanently yield or stretch. If it did, you will loose your preload and the fastener will eventually fail or fall out.
The coefficient of friction between a bolt and nut can vary tremendously. Fastener coefficients of friction vary as much as +/-20% of nominal. Therefore, if we used a value close to the proof load (i.e. 80%) and had a high variance in coefficient of friction (+18%) from our nominal calculation, we would end up going beyond the proof load and yielding the fastener, thus applying no preload.
The next thing you need to know is what value for the coefficient of friction you should use. If the fasteners are dry it can vary the coefficient of friction from 0.15 to 0.25. If either is lubed with some sort of lubricant (ie. dry film lube, WD-40), etc.), it changes the coefficient of friction quite a bit. We use 0.20 for the unlubed coefficient of friction and 0.09 for the lubed coefficient of friction. As you can see again, there is a pretty large difference, so please note if the fasteners you are about to put the torque wrench on are dry or lubed with any sort of lubricant.
Now you have all the tools to calculate the torque required, so here is the basic formula to use.
T = K x U x D x P where K is a constant that equals 1.33
U is the coefficient of friction
D is the basic diameter of the fastener
P is the preload you need
T is the torque required
Here's an example of where someone has a ½ inch diameter, unlubricated Grade 8 bolt and needs to calculate the torque setting.
D = 0.500 inch
U = 0.20 (unlubricated coefficient of friction)
K = 1.33 (constant)
As an example, a Grade 8 fastener has an ultimate strength of 150 ksi and a yield strength of 130 ksi (130,000 lb per square inch). The thread area (minimum diameter of the fastener) of a ½ diameter bolt is 0.1599 square inches according to MIL-S-8879C. Therefore, the full yield strength is 130,000 lb per square inch X 0.1599 square inches which equals 20,787 lb. 67% of the full yield strength is 13,927 lb.
Inputting this gives you T = (1.33) x (0.20) x (0.500 inch) x (13,927 lb) = 1852 INCH LBS or 154 FT LBS
Remember that this assumes you know what grade fastener you are using and whether or not it is lubricated. If we had chosen the lubricated case, the result is much different.
Here is the same example, but this time using a lubricated fastener.
T = (1.33) x (0.09) x (0.500 inch) x (13,927 lb) = 834 INCH LBS or 70 FT LBS
Notice that the torque required for a lubricated fastener is LESS THAN HALF that of an unlubricated fastener. I hope you note how important the coefficient of friction is and how it affects the required torque setting to achieve the same preload.
Posted April 27, 2007 - 09:36 PM
Posted April 27, 2007 - 09:51 PM
or am i totally just in outer space here?
Posted April 27, 2007 - 10:07 PM
> There will be fewer of them per inch
> They will be deeper
> The thread angle from the bolt axis will be lower
The first would reduce the thread area, but the second would increase it, so they would offset to some extent. I don't know how much.
The third, however, reduces the mechanical advantage of the thread. It takes more torque to achieve a given level of tension on the bolt with a coarse thread than with a fine thread, even though it takes less rotation to accomplish it.
I'm sure it needs to be considered.
Other than that, good stuff.
Posted April 27, 2007 - 11:37 PM
so i dunno about the whole 'MIL...' specs but i assume they are accounted for in that listing. plus, that info is from the rock-crawling community....their bolts are a lil bigger than ours. hahaha.
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