Is a rotation wrench necessary for mounting a 240D flywheel - PeachParts Mercedes-Benz Forum

otto huber · #1 02-27-2008, 06:02 PM

I was looking through the shop manual to find the torque specs on the flywheel bolts, and found that they suggest using a angle rotation wrench. Can I get by without it? Thanks,

Otto
'79 240D 4spd

Stevo · #2 02-27-2008, 06:13 PM

Quote:

Originally Posted by otto huber

I was looking through the shop manual to find the torque specs on the flywheel bolts, and found that they suggest using a angle rotation wrench. Can I get by without it? Thanks,

Otto
'79 240D 4spd

I dont but, Be careful, I've twisted off a couple of those bolts. Best not too reuse them.

Robert Squires · #3 02-27-2008, 06:35 PM

* No. When torque specs are given in terms of an initial torque AND an angle, you need to do the two steps, using the torque wrench for the initial torque and then using a "protractor torque" tool for the additional torque angle. Usually this tool is just used in conjunction with a long bar like a 1/2 in breaker bar.
* The stretch bolts have a calibrated neck so you can measure their diameter at the neck. The more the bolt has stretched, the smaller the neck will be. There are minimum diameter specs. But someone else recommended routinely replacing stretch bolts. I think that's good advice.

hoborobot69 · #4 02-27-2008, 06:50 PM

I disagree. an angle wrench will do the same as if you turn a bolt the specified angle. example: a manual specifies 235 ft lbs must be obtained then you must torque the bolt an additional 90 degrees. you dont need a specific wrench for this, mark the bolt on the side with a paint pen and mark the metal underneath so that these two markings are matching eachother, then turn 90 degrees...

Robert Squires · #5 02-27-2008, 10:16 PM

* The question was whether the "angular" or "protractor" torque was necessary. That is, could a ft-lb torque value be substituted for the angle? I don't know how you would convert, for example, a 90deg protractor torque to a certain number of ft-lb. That's why it's necessary to carry out the protractor or angle (semantics) torque.
* Whether you plot it with a school-kit protractor and use white paint dots or buy the protractor tool which snaps on the end of your ratchet is another question altogether.

Robert Squires · #6 02-27-2008, 10:26 PM

* My apology. I reread the thread title. The question really is about whether or not to get the protractor tool. Hoborobot69 is right. Paint dots will work but the tool allows for greater accuracy, particularly if you're torqueing to something off-quadrant like 135deg.

loepke72 · #7 02-27-2008, 10:47 PM

No, it really isn't possible to convert a certain number of degrees of rotation into a torque value because too many variables start to enter. The torque angle indicator really isn't necessary from a practical standpoint since it is pretty easy to figure out when a bolt has been turned 90°, and the tolerance on the angle is usually +/- 10° (if it's given). That said, it is still necessary to get the initial torque correct (the value given in N/m, ft/lb, etc.) before the "quarter turn" steps that stretch the bolt. For example, on the Ford 6.0L PowerJoke, the cylinder head bolt torque spec is: 65ft/lb, 85ft/lb, then three steps of 90°. That means tighten all bolts to 65ft/lb, then all of them again to 85ft/lb. At that point I mark a vertical line on all the head bolts so I can tell when they have been turned a quarter turn.

One more reason why angular rotation can't be equated to a torque value is normally it takes quite a bit of torque to stretch the bolts. I estimate 3-400ft/lb on the Ford example above. Also, when the bolt stretches, the rotational torque will level off somewhat. You can feel this when you're tightening a bolt a certain number of degrees. It feels like the bolt is softening and getting ready to break. A hard torque value for final tightening would likely result in broken bolts.

Number_Cruncher · #8 02-28-2008, 05:52 AM

Quite a lot of what has been written in this thread is absolutely true.

One thing to remember when installing bolts is that the aim is to stretch the bolt (usually an elastic, recoverable stretch, but sometimes a stretch where the bolt does actually yield, and will not return to its original length upon removal). Applying a torque is just a means to this end.

It is only by making the bolt longer that the bolt can actually clamp the joint together, and in many cases this clamp load is critical, because you want the joint to seal, or to resist slippage.

Pure torque tightening is at best a crude method of tensioning bolts. Even with top quality torque wrenches, new cut threads, top quality bolts and skilled fitters, you can expect a +/- 25% scatter on the actual clamp load you get.

Chief among the reasons for the scatter is variations in the amount of friction in the threads, and beneath the head of the bolt.

This uncertainty can be largely by-passed by using a torque + angle technique. The torque snugs down the joint, and the angle then applys a repeatable amount of stretch to the bolt, which in turn means that you get a more repeatable total tension in the bolt - which, remember is the aim of the excercise.

As the torque + angle will have been calculated using data on the geometry and stiffness of the bolts, the geometry and stiffness of the joint, and inherently reduces scatter, there's absolutely no way to go back and work out an equavalent torque value to use.

Even if you do three 90 degree turns with +/- 10 degrees tolerance on each, you will be applying a more consistent preload than if the joint were being tightened with a torque wrench alone (11% error rather than anywhere within the 25% error band I mentioned above)

If you use a protractor and do it properly, you are tightening the bolts with really quite a good accuracy.

In terms of deciding whether to re-use bolts or not, it depends what the FSM says about the particular bolt. Personally, I would prefer not to rely upon measuring diameters of bolts, because it needs an accurate measurement, as the reduction in diameter is a Poisson's ratio type of contraction and is much less than the corresponding change in length of the bolt - i.e., you need far lower measurement precision to determine if a bolt is OK if you measure length change.

An even better method of bolt installation which is beyond home use is where the bolt angle and torque are measured simultaneously during installation. By comparing changes in torque level as the angle turned increases, you can determine the point where the bolt begins to yield, and stop the machine there. This method both uses the bolt to its utmost, and has an extremely low scatter in terms of bolt pre-load.

leathermang · #9 01-18-2010, 12:31 PM

This is a really good thread....
Thanks for bumping it WHunter...