Re: Mercruiser Gimbal Questions.... noob
Since I'm on a roll on this thread, figured I would add this to Mischief:
Be careful torquing bolts. Automotive and marine practice seems to be to assume unlubricated conditions unless specified. The article below (I think it originally came from Rockcrawler.com) gives a pretty good overview of why it's important. If you use the unlubed torque on a lubed fastener it's pretty easy to yield the bolt. Note how the torque for the lubed fastener in the example is HALF the torque of the unlubed case.
BOLT TORQUE
You can under-torque or over-torque fasteners. Under-torquing a fastener is just as bad as over-torquing it. If the fastener is under-torqued, that will cause it to cycle many more times than normal and eventually shorten the fatigue life drastically, which in turn leads to an early failure.
Over-torquing risks shearing the fastener head off (break out the EZ out) or, even worse, it will actually yield the fastener. If the fastener yields, then there is NO preload on the fastener. No preload means the nut will most likely fall off due to vibration along with whatever it was supposed to hold on your vehicle.
Now, hopefully, you can see how important it is to torque the fastener to the correct level.
Put On Your Science Hat
There is some science behind how much to torque a fastener. 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??
Where I work, 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.
So now you are probably asking yourself right now then why don't we just use the proof load level for establishing the preload. You don't because of a little something called friction. For example, 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
Let's look at an example of where someone has a ? 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)
We need to calculate the preload P. Going back to my earlier Grade 8 versus Grade 5 fastener technical discussion, you can see that 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 ? 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 get 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
Always torque to and follow the instructions recommended by the manufacturer. If no torque instructions come with your hardware, contact the manufacturer for the appropriate torque levels. Since the coefficient of friction plays such a large role in the total torque required, make sure to use either clean dry threads or fully lubricated threads and use the corresponding coefficient of friction when calculating the total torque.
