Torque screw driver & flexible drive: will affect tighting torque?

[LPCapital]

Part of the timing procedure on a car I'll be working on requires to replace the hydraulic chain tensioner with a static one which needs to preloaded at 0.7Nm or 6 in lbs.

I found a torque screw driver that will allow me to be in that range:

The room around the area is too tight and the handle of the screw driver won't fit. I know u-joints cannot be used with torque setting tools because they affect the arm, thus change to torque, but will a flexible drive have a similar effect or such an extension will be torque neutral?

Here's the one I'm looking at:

 

[Art From De Leon]

Interesting, I wouldn't think so, BUT???

 

[skipskip]

I have NO basis for my theory, but that never stopped me before.

I think the spring might absorb some of the torque.

Also 6 inch pounds, is a VERY small amount of torque and hard to judge by "feel"


Skip

 

[krusty the clown]

I think the spring might absorb some of the torque.




Skip

yep............

 

[66HertzClone]

If you have a regular style inch pound torque wrench you could test it. Using the driver you posted along with the extensoin, torque a fastener down. Set the other wrench to the same setting and see what happens.

 

[forceyoda]

I would think that 6 inch pounds is such a low torque that it would not be enough to compress the spring of the extension.

 

[LPCapital]

I've checked with other people who have done the job and it appears the tool BMW provides is nothing more than a torque screwdriver with a flexible extension. Thinking on the way the flexible extension works it appears logic it won't affect torque. Thanks a lot for the input fellas. Although I don't post often I find this board to have a wealth of info!!!

 

[A_Pmech]

LP,

The flexible cable cannot "absorb" torque. It can absorb angular displacement in the form of torsion in the cable, however.

Also, as the bend radius of the flexible cable is reduced it will tend to "kink" under applied torque which can alter the moment arm though the cable and cause inaccurate torque readings. These readings generally understate the applied torque in my experience.

So long as the cable is used at a large bend radius it will perform accurately.

I've checked with other people who have done the job and it appears the tool BMW provides is nothing more than a torque screwdriver with a flexible extension. Thinking on the way the flexible extension works it appears logic it won't affect torque. Thanks a lot for the input fellas. Although I don't post often I find this board to have a wealth of info!!!

 

[LPCapital]

So long as the cable is used at a large bend radius it will perform accurately.

This does not apply to a u-joint, right? My understanding is that u-joints shoud never be used with a torque wrenches...

Thanks for the very detailed answer...

 

[A_Pmech]

LP,

Correct, a U-joint cannot be used with a torque wrench due to it's affect on applied torque. As a U-joint spins it's angular output velocity and thus output torque changes due to interaction of the two hinges making up the joint.

The amount of torque variation depends on the displacement of the joint. As a U-joint is displaced the output torque and speed becomes more and more dynamic.

For example, at 60 degrees joint displacement the joint will either double the applied torque and half the rotational velocity or, 90 degrees of rotation later, half the applied torque and double the rotational velocity. Somewhere between those two extremes, the joint will be "accurate" for a few degrees of rotation as the hinges "cancel" each other.

A flexible shaft does not suffer this problem as there is no hinged joint. So long as the flexible shaft remains un-kinked and is well-guided the torque applied at one end will be the torque delivered at the other end.

The law of conservation of energy applies, even to flexible extensions.

 

[LPCapital]

LP,

Correct, a U-joint cannot be used with a torque wrench due to it's affect on applied torque. As a U-joint spins it's angular output velocity and thus output torque changes due to interaction of the two hinges making up the joint.

The amount of torque variation depends on the displacement of the joint. As a U-joint is displaced the output torque and speed becomes more and more dynamic.

For example, at 60 degrees joint displacement the joint will either double the applied torque and half the rotational velocity or, 90 degrees of rotation later, half the applied torque and double the rotational velocity. Somewhere between those two extremes, the joint will be "accurate" for a few degrees of rotation as the hinges "cancel" each other.

A flexible shaft does not suffer this problem as there is no hinged joint. So long as the flexible shaft remains un-kinked and is well-guided the torque applied at one end will be the torque delivered at the other end.

The law of conservation of energy applies, even to flexible extensions.

Does that mean that the flexible drive is nothing else but a CV joint?

 

[Elroy]

The law of conservation of energy applies, even to flexible extensions.

Nice try but you're wrong.

You forgetting about the energy expelled in deflecting the spring. The input torque will be higher that the out put torque.

Would you like to try again??

 

[A_Pmech]

Does that mean that the flexible drive is nothing else but a CV joint?

A flexible drive is a constant velocity drive, yes. Although, it operates on the principle of transmitting torque though a cable of continuous cross-section. Thus, there are no uncancelled hinge moments.

 

[LPCapital]

You forgetting about the energy expelled in deflecting the spring. The input torque will be higher that the out put torque.

Isn't the spring deflected by you holding the torqe screwdriver in a particular position more so than by the torque applied to the screwdriver?

Considering the stiffness of the these spring and their resistance to wind/unwind it seems hard to believe...

 

[A_Pmech]

*AHEM*

The flexible cable cannot "absorb" torque. It can absorb angular displacement in the form of torsion in the cable, however.

Angular displacement times torque is energy, the last time I checked.

I'm pretty sure the takeup of energy in the form of torsion in the shaft conforms the the law of conservation of energy. The input torque will be equal to the output torque when the system is in balance and is no longer displacing.

Anyway, I believe we were discussing the application of ultimate torque though a torque-limiting screwdriver. Thus, energy absorbed by the shaft is of no concern here. The ability to transmit the ultimate torque without failing, or altering the torque though hinge moments is.

If you're going to try to discredit me, at least try to be on the same sheet of music. Re-read your physics while you're at it.

Nice try but you're wrong.

You forgetting about the energy expelled in deflecting the spring. The input torque will be higher that the out put torque.

Would you like to try again??

 

[mjozefow]

*AHEM*



Angular displacement times torque is energy, the last time I checked.

Anyway, I believe we were discussing the application of ultimate torque though a torque-limiting screwdriver. Thus, energy absorbed by the shaft is of no concern here. The ability to transmit the ultimate torque without failing, or altering the torque though hinge moments is.

I'm pretty sure the takeup of energy in the form of torsion in the shaft conforms the the law of conservation of energy...

If you're going to try to discredit me, at least try to be on the same sheet of music.

I would agree. Even if the shaft acts as a spring the reaction at the base (screw) must be the same as is being applied to the handle. Assuming it isn't accelerating radially, and the shaft is in a straight line. Maybe I should go pick up that statics book before I go further.


I don't see what conservation of energy has to do with in this instance though.

I mean yes, the shaft would act as a spring but as you stated this is of no consequence here. Still two more years of engineering classes to go so feel free to correct me.

 

[A_Pmech]

Mjozefow,

You're correct that the reaction at the screw must be equal to the reaction at the wrench when the system is not accelerating and therefore, by definition, in balance. That's Newton's 3rd law at work. "For every action there must be an equal and opposite reaction."

Regarding the law of conservation of energy, it was an attempt at humor and to illustrate how the output torque and rotational velocity of a U-joint are inter-linked, because the joint cannot create or destroy energy. Also, to illustrate that a flexible shaft operating in a steady state does not magically absorb torque. You're entirely correct that torque application is a static problem and not a dynamic one.

Elroy follows me around like a bad penny, doing his best to convince everyone else that I'm incapable of rational thought. He must have some obsessive-compulsive disorder, as he seems to find some joy in attempting to discredit me. Elroy likes to enter the clear waters of a discussion such as this one, generally involving me, then dump a load of raw **** into the water and shout, "Look what I've done!".

I find his outbursts humorous. However, they are disruptive and disrespectful to the original poster.

Good luck with your studies, Mjoze!

I don't see what conservation of energy has to do with in this instance though.

 

[Elroy]

The input torque will be equal to the output torque when the system is in balance and is no longer displacing.

Wrong

Would you like to try again??

This time you can keep the insults to yourself.

 

[mjozefow]

Perhaps you could give me an explanation then Elroy?

Not trying to join the fray, but you seem to be rather fervent that A_Pmech is wrong so I would like to know how his (and my) logic is in error.

Since you are an engineer by profession I would like to hear your side. I don't want to be giving others mis-information. But I don't see how the logic is wrong.

My two favorite threads are your Hein-Werner jack resto and A_Pmech's band saw resto. Obviously both of you are capable guys.

 

[mjozefow]

Mjozefow,

Regarding the law of conservation of energy, it was an attempt at humor and to illustrate how the output torque and rotational velocity of a U-joint are inter-linked, because the joint cannot create or destroy energy.


Good luck with your studies, Mjoze!

OK, and thanks!

 

[Wood]

You forgetting about the energy expelled in deflecting the spring. The input torque will be higher that the out put torque.

I'm not so sure. When you expell energy to deflect the spring, doesn't that energy stored in the spring also exert a force on the fastener that contributes to the total torque that the fastener 'experiences'? My physics is pretty rusty so I may not have all the terminology down, but I think the general idea is correct.

 

[Elroy]

Allow Elroy to simply point out that if you look at the free body diagram of the extension and you turn it, the forces are not balanced. The input torque in higher. That's why the dam thing turns.

All this **** about conservation of energy is just that. A bunch of **** designed to sound impressive.

And as far as the universal joint is concerned that is 50% ****. If you run two universal joints properly indexed in series the mechanical "advantage" developed by the first joint is going to be lost by the second joint. Commonly referred to as constant velocity u-joint.

Lets just say the **** was getting so deep that Elroy had to go get his hip boots on.

Would you like to hear a dissertation on Von Mises theory of shaft failure where the principle stresses are defined by Mohr's circle from the combined torsional and bending loads. Didn't think so.

So if you don't want to hear it from me why do you put up with this **** from others??

 

[mjozefow]

Allow Elroy to simply point out that if you look at the free body diagram of the extension and you turn it, the forces are not balanced. The input torque in higher. That's why the dam thing turns.

But isn't the final torque taken when the fastener comes to a stop? Thus balancing the torques?

Also, if the fastener is not accelerating, then wouldn't the torques be equal and opposite since there is no acceleration?


Not trying to start anything, I just don't want to spread mis-information if I'm wrong.

 

[gofastman]

Mjozefow,

You're correct that the reaction at the screw must be equal to the reaction at the wrench when the system is not accelerating and therefore, by definition, in balance. That's Newton's 3rd law at work. "For every action there must be an equal and opposite reaction."

Regarding the law of conservation of energy, it was an attempt at humor and to illustrate how the output torque and rotational velocity of a U-joint are inter-linked, because the joint cannot create or destroy energy. Also, to illustrate that a flexible shaft operating in a steady state does not magically absorb torque. You're entirely correct that torque application is a static problem and not a dynamic one.

Elroy follows me around like a bad penny, doing his best to convince everyone else that I'm incapable of rational thought. He must have some obsessive-compulsive disorder, as he seems to find some joy in attempting to discredit me. Elroy likes to enter the clear waters of a discussion such as this one, generally involving me, then dump a load of raw **** into the water and shout, "Look what I've done!".

I find his outbursts humorous. However, they are disruptive and disrespectful to the original poster.

Good luck with your studies, Mjoze!

Right, this is a simple ******* concept people, come on!

Wrong


This time you can keep the insults to yourself.

wrong

/facepalm

 

[mjozefow]

Right, this is a simple ******* concept people, come on!

wrong

/facepalm

Agreed, yet there is dispute. I'm trying to figure out why.

 

[gofastman]

Allow Elroy to simply point out that if you look at the free body diagram of the extension and you turn it, the forces are not balanced. The input torque in higher. That's why the dam thing turns.

All this **** about conservation of energy is just that. A bunch of **** designed to sound impressive.

And as far as the universal joint is concerned that is 50% ****. If you run two universal joints properly indexed in series the mechanical "advantage" developed by the first joint is going to be lost by the second joint. Commonly referred to as constant velocity u-joint.

Lets just say the **** was getting so deep that Elroy had to go get his hip boots on.

Would you like to hear a dissertation on Von Mises theory of shaft failure where the principle stresses are defined by Mohr's circle from the combined torsional and bending loads. Didn't think so.

So if you don't want to hear it from me why do you put up with this **** from others??

Agreed, yet there is dispute. I'm trying to figure out why.

I believe Mr. Elroy is confusing force and work

 

[Elroy]

I believe Mr. Elroy is confusing force and work

No look at this real close and tell us all who is confused here

Angular displacement times torque is energy, the last time I checked.

 

[Mike83]

Just tighten the screw til its tight and then give her another 1/2 turn. Jeezus

 

[A_Pmech]

I'd just like to point out two things to the crowd here:

1) How many times have you ever seen a mechanic use TWO U-joints at the same time? Did he bother to phase them and measure the angle of displacement of BOTH joints to ensure they would operate as a double-cardan shaft? I don't know about anybody else, but I'd have to be an octopus to make that work. That's why I didn't discuss the double-cardan shaft, It's of no practical application in the application of fastener torque.

2) Newton's Third Law still applies. When the system is in balance and not accelerating, input torque equals output torque, regardless of the rotational velocity. The extension cannot magically absorb torque when the system is in balance. This is why it is good practice to use a torque wrench with a smooth and even application of force, rather than yanking on the handle. NASA would love to negate the Third Law so they wouldn't have to equip astronauts with $30,000 impact guns to prevent them from spinning around in space!

Regarding Elroy:

The antics he started in this thread elicited a rather heated reply from me yesterday, which was deleted. I formally apologize to the members for that reply. While I did not cuss or insinuate anything, I did call the man a fool. I regret stooping to that level regardless of the accuracy (or inaccuracy) of my statements. Replies of that type reduce the quality of the discussions on this board. The post was also the subject of a PM from Ryan, the board admin. "Please be respectful or don't post." After a lengthy reply from me to Ryan, Elroy and I both received the following PM from Ryan yesterday during lunchtime:

Fellas... I don't really care who did what... I don't have an interest in that drama in the least. I really don't. It's childish in the context that this is a tool and garage forum. There should be no reason for ANYONE to get emotionally disturbed and upset for an opinion.

Seriously, think about it. It's ridiculous.

So, I'm swiping the sleight clean. I've got no problems with either of ya... Just please... I'm coming to you with hat in hand. Be respectful or don't post.

I'll leave it to the readers to decide if his post today is respectful and complies with Ryan's PM:

Allow Elroy to simply point out that if you look at the free body diagram of the extension and you turn it, the forces are not balanced. The input torque in higher. That's why the dam thing turns.

All this **** about conservation of energy is just that. A bunch of **** designed to sound impressive.

And as far as the universal joint is concerned that is 50% ****. If you run two universal joints properly indexed in series the mechanical "advantage" developed by the first joint is going to be lost by the second joint. Commonly referred to as constant velocity u-joint.

Lets just say the **** was getting so deep that Elroy had to go get his hip boots on.

Would you like to hear a dissertation on Von Mises theory of shaft failure where the principle stresses are defined by Mohr's circle from the combined torsional and bending loads. Didn't think so.

So if you don't want to hear it from me why do you put up with this **** from others??

With that, I have nothing further to add here. I feel what I've said above is respectful. I've called no names, used no foul language and insinuated nothing in this post. Just the facts.

I'd really like to leave all this **** behind. Really, I would. But, as you can see, the man just can't leave me alone, in this post, or any number of others! He doesn't know where to stop and let it go.

I do know where to stop. With this, I'll say no more on the subject.

 

[lametec]

There's always:

http://www.garagejournal.com/forum/profile.php?do=ignorelist

 

[MadMark]

Seems to me that if the bolt has stopped turning, even if the flex extension is twisted up, that by the laws of statics, the output (reaction) torque has to equal the input torque, as long as the bolt has stopped turning.

However, common sense seems to imply that if something is twisting, how could it be giving out the same torque that is being put in.

Also, if a pry bar is bending, is it transfering all of the force to the opposite end.
I'd love to know the answer to all this.

 

[WVU Tuba Dale]

To the OP: I think you'll be fine using the flexible extension.