Cam-out force – effect of spanner length, offset, head angle

[Samuel D]

A Hazet 610N (see pattern in second image) has the ring/box end perfectly in line with the handle. This means that there is no tendency for the spanner to slip off the nut when you pull the handle.

In contrast, the ring end of a Hazet 600N is angled 15 degrees to the axis of the handle. This feature, although sometimes useful, introduces a cam-out or cam-off force when the handle is pulled. Only friction keeps the spanner engaged on the nut. Additionally, the nut flats see uneven pressure along their six ‘lines’ of engagement, which means that as torque goes up the parts of the lines under most load deform sooner than they would with the Hazet 610N. There is also a force bending the nut on the bolt, which increases thread friction that acts against your intention of turning the nut.

Meanwhile the Hazet 630 has the ring ends almost parallel to the handle but at a deep offset. The offset introduces a cam-out force. However, the extra length of this spanner partially compensates for this, because a given nut torque is achieved with a lower force on the handle, and that lower force reduces the cam-out force too.

Similar points could be made about other spanner brands and models, Allen keys, sockets, torque wrenches, etc. For example, a shallow socket causes less cam-out force than a deep one, and a long (straight) ratchet causes less cam-out force than a short one.

Mechanics develop a feel to compensate for these forces, for example by pushing a deep-offset spanner forcefully onto the nut with their thumb near the head of the spanner while turning it with their palm, or by securing a socket with one hand while pulling the ratchet handle with the other.

I didn’t find much discussion about this topic with a search, maybe because the terms are vaguely used (including mine here, probably: cam-out as for a Phillips screwdriver, cam-off, or some other term?). The term offset, for example, is used by different people and companies for at least three different features of a spanner head.

I haven’t found anything useful in the few books on tools I’ve read either. I’m hoping John Deere Publishing’s Fasteners (Fundamentals of Service), which I ordered a couple of weeks ago from America, will cover some of this.

What have I missed? Where can I learn more?

 

[toolmutt]

It has been discussed before mostly in the "Do You Prefer a Wrench or Ratchet/Socket" threads. What you explained (better than I could) is why I always vote wrench in those threads.

 

[Samuel D]

Thanks. I still can’t find much, maybe because I’m searching for the wrong terms.

Have you come across any books that talk about these things? I’ve probably missed some details above.

 

[SeisMec]

FWIW Here are 18 pages on the Dynamics of Rotational Motion
http://d.umn.edu/~vvanchur/2017PHYS2017/Chapter10.pdf
that give the basic mathematics of calculating the applied torque at the connection of the wrench head to the fastener.

 

[toolmutt]

Is there an English version of ^that^?

 

[Samuel D]

FWIW Here are 18 pages on the Dynamics of Rotational Motion
http://d.umn.edu/~vvanchur/2017PHYS2017/Chapter10.pdf
that give the basic mathematics of calculating the applied torque at the connection of the wrench head to the fastener.

However, as far as I can see they don’t touch on the topic of discussion, that of cam-off forces arising from the wrench handle not being in the plane of the nut. (The 19° angle in example 10.1 is just rotational angle around the nut, not the offset we’re talking about.)

 

[CrazyTools]

It has been discussed before mostly in the "Do You Prefer a Wrench or Ratchet/Socket" threads. What you explained (better than I could) is why I always vote wrench in those threads.

I'm not sure a regular 7.5° offset wrench is ever really better than a 10" 3/8" Drive ratchet with a shallow socket in terms of cam out.

Most wrenches are also 12-point. Easier to cam out.

On a side note, I have the XBM wrench you're looking for.

 

[Spacey_G]

However, as far as I can see they don’t touch on the topic of discussion, that of cam-off forces arising from the wrench handle not being in the plane of the nut. (The 19° angle in example 10.1 is just rotational angle around the nut, not the offset we’re talking about.)

The same principles apply just in a different plane.

The tendency for a tool to slip off a fastener depends on the amount of torque driving it off the head. This is a torque perpendicular to the fastener axis, secondary to the loosening torque you're trying to apply. The slip-off torque is equivalent to the force applied to the wrench multiplied by the height of the offset.

You can reduce the tendency to slip off by reducing the offset (think shallow socket instead of deep socket) or reducing the force (for a given loosening torque, use a longer wrench).

You can also reduce the chance of slip-off by balancing the force by, say, supporting the ratchet head or using a T-handle instead of a single-ended tool. Compensation like that is something most people do naturally, especially using long extensions.

 

[Danglerb]

Cam out, as far as I understand it is MOSTLY an issue with a damaged fastener or tool, and not that much an issue with a wrench which slips off from user error.

The most offset wrench I can think of a lug wrench, and I don't recall any cam out issue as long as the wrench is fully seated on the lug.

 

[toolmutt]

Like Spacey G said, I think "compensation", or lack of it, is what leads to the arguments in threads here. Just my experience (I don't have a physics degree) but the farther away from the plane of the fastener that I apply my force, the greater the tendency for cam-off. I can usually compensate for that with my free hand but sometimes, due to space restrictions or length of reach, I may have to work one-handed. So as examples, the tendency for cam-off increases as one goes from (A.) a wrench where the handle to which I apply my force transfers that force to the ring directly around the fastener to (B.) a ratchet transferring the energy to the top off a shallow socket to (C.) a ratchet to deep socket to (D.) a ratchet to the drive end of a long extension on a socket. A technician who turns wrenches every day has a natural feel for that compensation. An infrequent DIYer has to try a little harder to prevent cam-off. I also think that the amount of chamfer on the socket or wrench plays a role. A really deep chamfer equals less contact. Just my humble, uneducated opinion.

 

[SeisMec]

Is there an English version of ^that^?

See Spacey_G below.

However, as far as I can see they don’t touch on the topic of discussion, that of cam-off forces arising from the wrench handle not being in the plane of the nut. (The 19° angle in example 10.1 is just rotational angle around the nut, not the offset we’re talking about.)

Sorry, I should have taken a closer look at the examples. But as Spacey_G says — with a few technical substitutions —

The same principles apply just in a different plane.

The tendency for a tool to slip off a fastener depends on the amount of t̵o̵r̵q̵u̵e̵ force driving it off the head. This is a t̵o̵r̵q̵u̵e̵ force perpendicular to the fastener axis, secondary to the loosening torque you're trying to apply. The slip-off t̵o̵r̵q̵u̵e̵ force is equivalent to the force applied to the wrench multiplied by the height of the offset.

For the sake of exposition, suppose you have 2 inch tall deep socket at the end of a 12" (1 foot) long breaker bar @ 90° to the bolt head. If you apply 30 lb of force to the very end of the breaker bar, the total force is 30 lb/ft. The approximate force tending to cam the socket off the bolt head is 2" socket height / 12" handle length * 30 lb/ft = 5 lb/ft.

And the approximate actual turning force (TORQUE) delivered to the bolt head is 30 lb/ft - 5 lb/ft = 25 lb/ft. In the real world, the only number you're ever likely to care about is the last one — delivered torque to the head of the bolt.

 

[bonneyman]

I've noticed that and mentioned it, but not with the eloquence of your post.

It's why I prefer long wrenches with zero offset, and shorty wrenches when theres any substantial offset. There are exceptions, of course, but this is my preferred choice.
The shorter ones are typically lower torque situations and my hand almost completely covers the whole shank, so, tipping is more controllable.

 

[Spacey_G]

See Spacey_G below.



Sorry, I should have taken a closer look at the examples. But as Spacey_G says — with a few technical substitutions —



For the sake of exposition, suppose you have 2 inch tall deep socket at the end of a 12" (1 foot) long breaker bar @ 90° to the bolt head. If you apply 30 lb of force to the very end of the breaker bar, the total force is 30 lb/ft. The approximate force tending to cam the socket off the bolt head is 2" socket height / 12" handle length * 30 lb/ft = 5 lb/ft.

And the approximate actual turning force (TORQUE) delivered to the bolt head is 30 lb/ft - 5 lb/ft = 25 lb/ft. In the real world, the only number you're ever likely to care about is the last one — delivered torque to the head of the bolt.

You're incorrect to substitute 'torque' for 'force'. It's still a torque tending to cam the socket off the bolt (notice your units!) it's just acting in a different direction than the torque you care about.

Maybe it would be better to use the term 'moment' for the camming action. 'Loosening torque' and 'camming moment'. Same units of force x distance, but they're in different directions and act on the bolt and socket differently.

Additionally, it's incorrect to subtract your 5 ft-lbs of camming moment from your 30 ft-lbs of loosening torque. Those are two different moments acting in different directions at the same time. They do not add or subtract. You'll be loosening the bolt with 30 ft-lbs while simultaneously trying to cam the socket off the bolt head with 5 ft-lbs.

 

[Handyandy23]

Kind of the unknown variable here too is how much force is required to cause cam-out. A lot of that comes down to the fit of the socket / wrench on the fastener, and the material the fastener is made out of.

If I have a an old rusty bolt that I have to hammer the socket onto, chances are it isn't going to cam-out without a lot of 'camming moment'. If the bolt head is brand new, torqued to a high number, and I'm using a cheap socket that doesn't have the greatest fit, it's going to require a lot less camming moment to actually cause cam-out.

Depending on the circumstances and the quality / fit of the socket or wrench, a tool with a lower offset might result in more cam-out than a better quality tool with a higher offset.

 

[Samuel D]

This is an old thread, but I’d still like to add three things:

1. The book I ordered, John Deere Publishing’s Fasteners (Fundamentals of Service), while interesting in many ways, did not improve my understanding of this particular topic.
2. A correctly used sliding T-bar and socket will produce zero camming moment, to use Spacey_G’s term. That makes it a valuable tool for some fasteners. (Good mechanics will know this anyway.)
3. My thanks to the participants who raised some interesting points I hadn’t considered.

 

[M6erfan]

This is an old thread, but I’d still like to add three things:

1. The book I ordered, John Deere Publishing’s Fasteners (Fundamentals of Service), while interesting in many ways, did not improve my understanding of this particular topic.
2. A correctly used sliding T-bar and socket will produce zero camming moment, to use Spacey_G’s term. That makes it a valuable tool for some fasteners. (Good mechanics will know this anyway.)
3. My thanks to the participants who raised some interesting points I hadn’t considered.

The sliding T bar is underrated here in the U.S. Personally, I find them very useful

 

[Samuel D]

The sliding T bar is underrated here in the U.S. Personally, I find them very useful

Yeah. With the socket centred, a sliding T-bar works like a tap wrench in that it can deliver pure torque if the user is careful to balance the forces on the two handles. So no bending of the fastener axis or risk of the socket camming off.

 

[Spacey_G]

Using a T-handle is something I mentioned earlier in the thread. It allows you to apply zero net force while applying non-zero torque.

 

[Samuel D]

Using a T-handle is something I mentioned earlier in the thread.

So you did. Sorry I missed that on this quick second reading tonight.

I’m still interested in books anyone knows that cover this topic in more depth. There have been enough times I’ve thought I’ve known something inside-out, only to discover whole new facets of subtle understanding later.

 

[Spacey_G]

Samuel_D - They're unlikely to cover socket camout specifically, but you might be interested in texts covering engineering statics. Those are the principles that inform the interactions between a socket and a fastener. At least to the point of load determination. A popular one is Hibbeler's Engineering Mechanics: Statics.

If you can get comfortable with drawing free-body diagrams for every component in a mechanical system, you'll have a good basis for understanding why (non-moving) systems behave the way they do.