How do I use a turnbuckle effectively?

[DaveC426913]

TL;DR Summary

Speciifically, how much thread needs to be ... threaded, to function within operational parameters?

I bought four stainless steel turnbuckles to replace the vernier adjusters on the shrouds of my sailboat.

The cotter pin diameter is 1/4" - the same as what I am replacing. I was told that that is the metric by which one judges the capability of a turnbuckle.

They have a 1 1/2" long thread on each end and so can range from 6" inches to 9". Naturally, their functional range is less than 9". But my question now is: how much less?

On a test run, one of them blew out and almost dismasted me.

I checked once safely back ashore - the threads are stripped.
The left one is brand new. The right one looks like its threads are all stripped.
That's after one hour of sailing.

Before I go back to the chandlery and complain it's a hardware fail, I have to be sure I didn't install them incorrectly.

I had made sure I had what I thought was enough turns. It was at least 5/8", if not 3/4" of an inch on each end. If that's not enough, that means the useful range of these turnbuckles tops out at 7 1/2" at best - barely 1 1/2" of range. I don't really have enough play in my shrouds to go much further than I have.

If I go a size up in turnbuckles, the next ones are too long - longer than the distance from tang to shroud swage, and I would have to remove and completely replace my shrouds.


What can I do?

 

[jack action]

Looking at typical turnbuckles, you can deduce that approximately twice the diameter should be enough:

source: https://duckduckgo.com/?t=ffab&q=turnbuckle&iax=images&ia=images

To have a more precise number:

Recommended thread engagement​

While making a connection, the bolted joints should be designed so that the screw breaks before the threads strip. For components with tapped holes, the length of engagement (the number of threads engaged between the tapped hole and the screw) should be adjusted to favor this criterion. Table 1 summarizes the general rule of thumb for thread engagement distance calculation.

Table 1: General rule of thumb for thread engagement distance calculation

Material	Minimum thread engagement distance
Steel	1.0 to 1.5 x base diameter of the screw or bolt
Cast iron, brass	1.5 to 2.0 x base diameter of the screw or bolt
Aluminum	2.0 to 2.5 x base diameter of the screw or bolt
Soft materials (like plastic)	The minimum thread engagement distance will be higher and require trials to establish the optimum joint strength.

And then for even more precision, you can do the math:

Where:

LE2 = Minimum thread engagement length (tensile strength and shear area ratio)
UTS_external = Ultimate tensile strength of external thread material
UTS_internal = Ultimate tensile strength of internal thread material
AS_s = Minimum thread shear area for external threads
AS_n = Minimum thread shear area for internal threads
n = # of threads per inch
LE = Length of thread engagement
d_1_min = Minimum major diameter of external thread
d_2_min = Minimum pitch diameter of external thread
D_1_max = Maximum minor diameter of internal thread
D_2_max = Maximum pitch diameter of internal thread
A_s = Tensile shear area
d = Basic major diameter of external thread (in, mm)

source: https://www.omniamfg.com/mechanical...ate-fastener-minimum-thread-engagement-length

 

[Baluncore]

I have been told that full strength is obtained with seven turns. But with fine threads, I prefer to use one outer diameter, the length of a nut.

The threads on the rods were probably rolled, not cut, and so will be rounded and under-size at the ends. You might need to cut off the rolled end with a cutting disc to get a constant full thread OD. That will allow closer contact between the screw ends, with good thread when short. If poor LH and RH threads at the ends meet, they will reduce the range of adjustment.

Lateral vibration of a tight stay in the wind will produce very high axial forces on the turnbuckle, due to the triangle of forces being long and thin.
This is also a special case since each lock-nut is working with the rope tension to strip the thread.

If you adjust the turnbuckles under load, without lubrication or an anti-seize compound, the stainless steel threads will probably gall. That may remove some of the thread OD material, just where it is most needed. That could generate the damage you show.

 

[erobz]

On a test run, one of them blew out and almost dismasted me.

It almost dismasted you or the boat???

 

[DaveC426913]

Looking at typical turnbuckles, you can deduce that approximately twice the diameter should be enough:

If that's the case, I was well within the operational range at 5/8"- 3/4".

 

[DaveC426913]

It almost dismasted you or the boat???

If it fell the wrong way, it might have dismasted me too!

 

[DaveC426913]

I have been told that full strength is obtained with seven turns. But with fine threads, I prefer to use one outer diameter, the length of a nut.

I was still well within that.

The threads on the rods were probably rolled, not cut, and so will be rounded and under-size at the ends. You might need to cut off the rolled end with a cutting disc to get a constant full thread OD.

Interesting you mention this. My next question was going to be: should the bolt be loose in its socket?
There's definitely a little play in these turnbuckles. I can wiggle them around a tiny bit, at almost any extension.

My naive thought when I observed this was 'Hey, doesn't that mean the threads are not fully overlapped? Isn't that weak?' But who was I to argue with marine-grade hardware?

That will allow closer contact between the screw ends, with good thread when short. If poor LH and RH threads at the ends meet, they will reduce the range of adjustment.

The screws don't touch in the middle. There's a hole for a lever and locking wire.
The shaft is 4" long and the screws have 1 1/2" of thread.

I guess I could cut 1/4" - 3/8" off, but that doesn't solve my problem, does it? I don't have the option of screwing the turnbuckle in more than it is now. All I would be doing is cutting off useless thread.

Lateral vibration of a tight stay in the wind will produce very high axial forces on the turnbuckle, due to the triangle of forces being long and thin.
This is also a special case since each lock-nut is working with the rope tension to strip the thread.

I think this is exactly what happened.

If you adjust the turnbuckles under load, without lubrication or an anti-seize compound, the stainless steel threads will probably gall. That may remove some of the thread OD material, just where it is most needed. That could generate the damage you show.

I adjusted them at-dock and only hand-tight. Nowhere near as tight as the shrouds are recommended to be.

In fact, one of my theories was whether not having the rigging tight enough could have caused enough wobble to wear the hardware.


My take-away here is that - despite being sold in a marine chandlery, where they tend not to cheap out on hardware - these are cheap turnbuckles, and failed for that reason.

 

[Baluncore]

Interesting you mention this. My next question was going to be: should the bolt be loose in its socket?
There's definitely a little play in it. I can wiggle it around a tiny bit, at almost any length.

If the stainless thread is a tight fit, it will gall.

My turnbuckles never have sufficient range. I would like one thread to be cut on a larger tube, sufficient to house the other thread, so the ends don't meet and stop. Another way would be to stagger the threaded rod axes, so they pass each other in the middle of the turnbuckle.

I would first assemble rigging by hand, then while sailing, take up floppy slack by hand, when the shroud is on the leeward side.

Take care, the term "shroud" has another meaning.

 

[DaveC426913]

Interesting. That turnbuckle I posted a pic of is selling for $75.

The ones I bought are $14.

In fact, my 20/20 hindsight is reminding me that the reason I didn't do this upgrade years ago is because I was looking at $75 turnbuckles, and this time my critical judgement failed me and I bought impulsively,.


This site has the exact size and type, and calls them "Economy". "600-lb working load"
https://ca.binnacle.com/Turnbuckles-&-Accessories/c147/p11499/Stainless-Steel-Economy-1/4"-Turnbuckle-Tubular-Jaw-&-Jaw/product_info.html

 

[Flyboy]

Lemme make sure I understand this setup correctly: these turnbuckles are adjusted at the dock/while moored and not while under load, and are being used to make fine adjustments before/after sailing?

Given the specified thread engagement depth and the diameter, and the quoted price, I think the hypothesis that it just overloaded the material is the most likely one.

 

[DaveE]

despite being sold in a marine chandlery, where they tend not to cheap out on hardware - these are cheap turnbuckles, and failed for that reason.

I always wanted "name brand" hardware on my boat for anything that really shouldn't break. You get what you pay for, there's lots of cheap stuff in that market because the good stuff is expensive. It's a small market with special requirements. I see a lot of turnbuckles like that used for deck railings and such in building construction; applications that can get away with poor quality strength wise.

I would also suggest going to a local boatyard and talking to a pro rigger. They know what fails and what doesn't. They see many more boats than we ever will.

 

[Rive]

Ugh.

The turnbuckle up there looks like one for fences and similar stuff.

I think what you need is more like the ones for agriculture/tractor linkage. Very different. The thread profile too.

 

[sandy stone]

In particular, for a sailboat I would try to find some open-barrel turnbuckles, so you know for sure how much thread engagement you have. Possibly they might be a little longer, too.

 

[jack action]

My next question was going to be: should the bolt be loose in its socket?
There's definitely a little play in these turnbuckles. I can wiggle them around a tiny bit, at almost any extension.

You have jam nuts. Use the jam nuts. More in the following video about jam nuts and lock wires:

​

 

[sophiecentaur]

l also suggest going to a local boatyard and talking to a pro rigger.

That's probably the best advice so far. But a good chandlery shop should be able to sell you the right turnbuckle for length and quality for the rigging. (There is some rubbish available on line - as with all 'marine grade' parts)
I would expect that the standing rigging would be of the right length for the boat. If it's too short then why not insert a strop or some extra links? If the stay is too long then you'd need to have it shortened (with the proper suaging press).
Just a long shot but are the two lengths of thread equal, inside the turnbuckle? i.e. full length of thread engaged at both ends?

And another thing: are all the threaded pieces from the same original turnbuckle? (i.e. all the threads actually match and not a hybrid fixing)

 

[jack action]

They have a 1 1/2" long thread on each end and so can range from 6" inches to 9". Naturally, their functional range is less than 9". But my question now is: how much less?

This site has the exact size and type, and calls them "Economy". "600-lb working load"
https://ca.binnacle.com/Turnbuckles-&-Accessories/c147/p11499/Stainless-Steel-Economy-1/4"-Turnbuckle-Tubular-Jaw-&-Jaw/product_info.html

In this link, it says that the maximum length is 8½" eye-to-eye. There is your answer about minimum length engagement: ¼".

 

[DaveC426913]

You have jam nuts. Use the jam nuts. More in the following video about jam nuts and lock wires:

​

I did use the jam nuts.
And the turnbuckle failed; it didn't unscrew, so the lock wire isn't involved.

 

[DaveC426913]

I would expect that the standing rigging would be of the right length for the boat.

The vernier adjusters that are stock for the boat have a lot more freedom in their length.

If it's too short then why not insert a strop or some extra links?

Yes, I had to cut down a vernier and use it as an extension. Which means I haven't eliminated the vernier adjusters, I've got vernier adjusters AND turnbuckles...

If the stay is too long then you'd need to have it shortened (with the proper suaging press).

Right. I would need to remove the standing rigging to do so. Taking the boat out of commission.
And it's not a good idea to shorten a rig that's 24 years old. I would replace it.

Just a long shot but are the two lengths of thread equal, inside the turnbuckle? i.e. full length of thread engaged at both ends?

Yes. I make sure to entirely unscrew them myself and re-assemble.

And another thing: are all the threaded pieces from the same original turnbuckle? (i.e. all the threads actually match and not a hybrid fixing)

Well, all bets are off when it comes to this cheap hardware, aren't they?

 

[DaveC426913]

In this link, it says that the maximum length is 8½" eye-to-eye. There is your answer about minimum length engagement: ¼".

Sorry, it is only an estimation of the total length. I wouldn't calculate the minimum operating length based on gross measurements.

(It would be like saying "A standard Chevy Bel-Air is exactly 4'8" at the roof line. Proper tire pressure can be achieved by inflating the tires until your roof is 4'8". " )

 

[DaveC426913]

Look at the difference in thread length (and thus range) and in thread fineness (and thus strength):

And the premium turnbuckle only needs 1/4" of thread engagement because it's so strong.

I think Baluncore was spot-on when he suggested the "Economy" one was rolled steel rather than cut, and therefore the threads are poorly-formed and shallower. They can just pull right through under load.

 

[DaveC426913]

I always wanted "name brand" hardware on my boat for anything that really shouldn't break. You get what you pay for, there's lots of cheap stuff in that market because the good stuff is expensive. It's a small market with special requirements. I see a lot of turnbuckles like that used for deck railings and such in building construction; applications that can get away with poor quality strength wise.

Yes, I was in a Marine shop (check), and these are what they had (check). It simply didn't occur to me that there would be light duty and heavy duty hardware - or that a Marine shop would have light duty on the shelf and not label them as such.

"Oh what mangled fruits we reap
When we first try to buy cheap!"
(I just made that up.)

And the fruits I reaped were five stitches in my arm and almost having my SeaSaw dismasted.

 

[DaveC426913]

Lemme make sure I understand this setup correctly: these turnbuckles are adjusted at the dock/while moored and not while under load, and are being used to make fine adjustments before/after sailing?

Well, they coud be adjusted under load. But really they should be set to provide a specified tension of the shrouds when not under load.

Given the specified thread engagement depth and the diameter, and the quoted price, I think the hypothesis that it just overloaded the material is the most likely one.

Yes.

 

[Baluncore]

Stainless steel is hard to cut with steel tools because it rapidly work-hardens. That is especially true of marine grade 316. Grade 304 is designed to be easier to machine, but is not as resistant to chlorine, so 304 is more suited to inland waterways. The machining of stainless steel has become easier since the advent of carbide and ceramic threading inserts.

You can recognise a rolled thread because, being a deformed surface, the thread crests will have a slightly greater diameter than the shank. The rolled thread root is also more rounded, which reduces stress concentration at the critical minimum diameter.

Look at the difference in thread length (and thus range) and in thread fineness (and thus strength):

The two turnbuckles, shown in that post #20, appear to have thicker shanks, so were probably cut in an NC lathe.

Both machine cut, and rolled threads, have an initial taper at the end of the threaded rod. That makes it easier to align and start the thread in a nut, but by reducing the height of the tooth, it reduces the strength of the thread at the end of the rod. I believe that is what caused the failure in the OP case.

 

[DaveC426913]

You can recognise a rolled thread because, being a deformed surface, the thread crests will have a slightly greater diameter than the shank. The rolled thread root is also more rounded, which reduces stress concentration at the critical minimum diameter.

I don't quite understand the details but it sounds like you're referring to the bolt rather than the body of the turnbuckle. It is the body that has stripped threads. The bolt seems to have fared all right.

The two turnbuckles, shown in that post #20, appear to have thicker shanks, so were probably cut in an NC lathe.

?
One them is $75, the other is $14. Do you mean you think they are both manufactured the same way?

Or am I misunderstanding what you mean by "the two turnbuckles"? Do you perhaps mean "the two bolts of the $75 turnbuckle"?

Both machine cut, and rolled threads, have an initial taper at the end of the threaded rod. That makes it easier to align and start the thread in a nut, but by reducing the height of the tooth, it reduces the strength of the thread at the end of the rod. I believe that is what caused the failure in the OP case.

I think so too. The $14 one is quite loose at the first few threads. But I assumed that that's why I was making sure I had at least 5/8" - 3/4" threaded on.

The $75 one actually has significantly less thread contact. The overlap area is fixed, at about 1/4" - much less than the $14 one had.

 

[Baluncore]

I don't quite understand the details but it sounds like you're referring to the bolt rather than the body of the turnbuckle. It is the body that has stripped threads. The bolt seems to have fared all right.

I refer to the bolt. If the bolt threads are undersize, with crests removed at the end, they will contact less of the body threads, and the narrow tops of the complete body-threads may then be pulled out, by the thicker remaining bottoms, of the bolt threads.
That takes some thinking about, but it does explain why a good internal thread can be stripped by an undersize bolt, one that is missing the crests to make it easy to start.

One them is $75, the other is $14. Do you mean you think they are both manufactured the same way?

I think the bolts in both were threaded by machine cutting, but I cannot see diameters clearly in the photo. You need to compare the thread diameters to the shank diameters. Rolled threads use less material than cut threads, because the rolled shank is thinner.

Or am I misunderstanding what you mean by "the two turnbuckles"? Do you perhaps mean "the two bolts of the $75 turnbuckle"?

A turnbuckle has two opposite threaded bolts, I refer to the 4 bolts in two turnbuckles.

The $75 one actually has significantly less thread contact. The overlap area is fixed, at about 1/4" - much less than the $14 one had.

In either case, the length of thread contact will be sufficient.

The $14 one has a tube with a thread in each end. The tube has been shrunk down at the end before being threaded. The actual thread will not go far up the tube, before the tube cavity opens out. That reduces thread friction, and prevents accumulated pitch errors causing problems, should the bolt stretch, or be bent slightly.
The internal thread need not be "cut" with a tap, it may have been deformed by a "threadflow" tap, which is an equivalent process to thread rolling on a bolt.

 

[DaveE]

Well, they coud be adjusted under load. But really they should be set to provide a specified tension of the shrouds when not under load.

Unless you're a Pro. Then your sailmaker, and maybe your rigger, does the fine tuning underway. The first is good enough for a reliable boat and is a cookbook procedure, the second is about optimization and spending money to win races.

 

[DaveC426913]

Unless you're a Pro. Then your sailmaker, and maybe your rigger, does the fine tuning underway. The first is good enough for a reliable boat and is a cookbook procedure, the second is about optimization and spending money to win races.

Sure. But not for this boat.

This boat doesn't have specs for such things.

Its a bastardization of sail and power and does neither very well, but it is very accommodating for those who want both.

The manual says "about yea tight".

 

[DaveC426913]

I refer to the bolt. If the bolt threads are undersize, with crests removed at the end, they will contact less of the body threads, and the narrow tops of the complete body-threads may then be pulled out, by the thicker remaining bottoms, of the bolt threads.

I hadn't really noticed a tapering of the bolt ends.

That takes some thinking about, but it does explain why a good internal thread can be stripped by an undersize bolt, one that is missing the crests to make it easy to start.

It would, yes.

I think the bolts in both were threaded by machine cutting, but I cannot see diameters clearly in the photo. You need to compare the thread diameters to the shank diameters. Rolled threads use less material than cut threads, because the rolled shank is thinner.

I guess I could get pics, but not for a week. The only ones I haven't returned are currently holding my mast up.

A turnbuckle has two opposite threaded bolts, I refer to the 4 bolts in two turnbuckles.

Yes I just don't know which two turnbuckles. There's one $14 tb and one $75 tb. I guess you're saying they're made the same way.

In either case, the length of thread contact will be sufficient.

The $14 one has a tube with a thread in each end. The tube has been shrunk down at the end before being threaded. The actual thread will not go far up the tube

Oh. OHHHH.

I just assumed the thread ran up through entire barrel. You're saying the closed barrels are only threaded for the first fraction of an inch. Just like the open barrel ones do.
Ohhhh.

, before the tube cavity opens out. That reduces thread friction, and prevents accumulated pitch errors causing problems, should the bolt stretch, or be bent slightly.

Ah yes. That makes sense. If the thread contact is too long it will bind easier.

So the $14 turnbuckle might only have a similar 1/4" or so area of threading.

I was wrong : as it is threaded in, its strength does NOT keep increasing; it tops out at some point. And that's at 600lbs.(ish).
Got it.

 

[Baluncore]

For maximum adjustment range, turnbuckles are usually made with the two bolts, threaded long enough to meet in the centre. In the case of the tubular turnbuckle, the bolts should stop when they reach the hand turning rod, that passes through the hole, across the middle of the barrel.

I just assumed the thread ran up through entire barrel. You're saying the barrels are only threaded for the first fraction of an inch. Ohhhh.

If the thread went all the way up the barrel, it would take more work to manufacture, and there would be confusion when one thread met the other, with the opposite hand, coming the other way. Like the bindweed and the honeysuckle.
Michael Flanders and Donald Swann. 'Misalliance'. 1957

 

[DaveC426913]

For maximum adjustment range, turnbuckles are usually made with the two bolts, threaded long enough to meet in the centre. In the case of the tubular turnbuckle, the bolts should stop when they reach the hand turning rod, that passes through the hole, across the middle of the barrel.

Yes.

If the thread went all the way up the barrel, it would take more work to manufacture, and there would be confusion when one thread met the other, with the opposite hand, coming the other way.

Well, I didn't assume it went all the way to the centre. But I did assume it went essentially as far as the bolt could reach.

 

[Tom.G]

You have jam nuts. Use the jam nuts. More in the following video about jam nuts and lock wires:

​

As an obvious point for those that have not watched the video (time 2:40):
Jam nuts ADD to the strain on the threads, making them more likely to strip/fail.
Please keep it in mind!

Cheers,
Tom

 

[DaveE]

OK, I tried google and failed; honestly, I tried. What is a SeaSaw? Bonus points for a picture.
I'd trade you for a picture of my last boat, a C&C 33, but you could just google that.

 

[Baluncore]

Teeter Totter ?
https://en.wikipedia.org/wiki/Seesaw

 

[sophiecentaur]

or maximum adjustment range, turnbuckles are usually made with the two bolts,

That's not the only reason . Having a LH and RH thread means you don't need a swivel in the rigging; the only thing that rotates is the body of the turnbuckle.

 

[sophiecentaur]

Jam nuts ADD to the strain on the threads,

My turnbuckles used a split pin through the tip of each bolt which wouldn't turn once it met the frame of the buckle (no stress on the bolt at that point). Not very strong and you could bend the pin but I never found one that undid itself.