Replacing the actuator

[ninhalo5]

Hi, Got a dumb question.

I'm getting ready to put on a new actuator and the instructions recommends using 3, 1/2" X 4" grade 5 bolts. Wouldn't it be better to use 6, 1/2" X 1-1/4" grade 8 carriage bolts?

I would think that would give more strength. The longer the bolt the higher in flexibility and more of a chance to crack under pressure. I would rather have 6 bolts cause if 1 broke for whatever reason you would still have 5 bolts holding it on.

Or am I over thinking this?

Any one have a thought?

Thanks

 

[bruceb58]

Re: Replacing the actuator

Either are fine. Some might say that Grade 8 are more brittle because they are harder but not likely a problem for this app. Not a mechanical engineer but that is my understanding.

 

[Bondo]

Re: Replacing the actuator

Hi, Got a dumb question.

I'm getting ready to put on a new actuator and the instructions recommends using 3, 1/2" X 4" grade 5 bolts. Wouldn't it be better to use 6, 1/2" X 1-1/4" grade 8 carriage bolts?

I would think that would give more strength. The longer the bolt the higher in flexibility and more of a chance to crack under pressure. I would rather have 6 bolts cause if 1 broke for whatever reason you would still have 5 bolts holding it on.

Or am I over thinking this?

Any one have a thought?

Thanks

Ayuh,.... Carriage bolts aren't made in gr.8, just gr.2, 'n gr.5...

Yer gonna have a 'ell of a time gettin' short bolts into their holes, tryin' to reach in through the end of the neape tube...

The Long bolts recommended are only seein' a shear factor, right at both ends, No stresses in the middle...
'n with an 80,000 lb Shear point, the long bolts will never be stressed, anywhere near their limits...

So,.... Yes, yer over-thinkin' it....

 

[dingbat]

Re: Replacing the actuator

I would think that would give more strength. The longer the bolt the higher in flexibility and more of a chance to crack under pressure.

Your assumption about the longer bolts is incorrect. In this assembly, there are both shear and rotational forces at work. The shear load on each bolt would be identical if you used 3 or 6 bolts however……. I think you will find there are spacers over the bolts inside the tube. The spacer ties the two sides of the tube together creating a cross section with a significantly higher ability to resist rotation than the tubing alone.

 

[smokeonthewater]

Re: Replacing the actuator

think of this too.... in a few years you need to remove them for some reason but they are rusty.... one case you put a wrench on one end and a ratchet on the other and give em hell, The other case, some of the carriage bolts strip inside and you have to cut em off.

Even without spacers, the 3 bolts are the better choice and grade 5 is the better choice for durability here.

 

[Frank Acampora]

Re: Replacing the actuator



I always use either 4 or 6 short bolts on my plain couplers(depending upon trailer capacity). Reason: Using the long bolts you must use self-locking nuts. You can not torque the long bolts down correctly without starting to deform the trailer tongue and coupler. Do not use carriage bolts. You need to use two wrenches.

With the short grade 8 bolts, you can use either self-locking or regular nuts with lock washers. You can also fully torque them with no danger of deforming the tongue because you are not squeezing it. AND, the nuts are all inside the tongue, leaving the outside looking neater.

 

[ninhalo5]

Re: Replacing the actuator

Hmm, interesting. Well I still have an assortment of grade 8 carriage bolts that came with my receiver (or at least they claim to be 8) and some of the holding plates for them. I figured it might be better to use them instead of the long bolts, but I see everyone's points.

One thing I don't understand, why are grade 5 bolts the better choice for the actuator, when the strength of an 8 is way higher and is recommended for the receiver. Yield strength is 81k psi compaired to 130k psi.

 

[dingbat]

Re: Replacing the actuator

One thing I don't understand, why are grade 5 bolts the better choice for the actuator, when the strength of an 8 is way higher and is recommended for the receiver. Yield strength is 81k psi compaired to 130k psi.

The grade 8, 130K psi fastener is superior to the grade 5 in all aspects except price. In your case, the forces on the coupler bolts are next to nothing and don't warrant the use of the more expense grade 8 bolts. If you already have them, use them.

 

[UncleWillie]

Re: Replacing the actuator

... Yes, yer over-thinkin' it....

+1

80,000psi, 130,000psi, The tongue only sees a few hundred pounds!

4 or 6 of ANYTHING will still be way overkill!

 

[Silvertip]

Re: Replacing the actuator

+1

80,000psi, 130,000psi, The tongue only sees a few hundred pounds!

4 or 6 of ANYTHING will still be way overkill!

That couple of hundred pounds of force would be under static load (as vehicle stopped). Shock loading is much higher (such as hitting a pot hole) but still well under the 80,000/130,000 numbers. Carriage bolts are also not used unless the bolt holes have a square for the anti-rotation square to sit in. I think machine bolts would be the correct terminology here.

 

[Frank Acampora]

Re: Replacing the actuator

Grade 5 is better because when they rust solid, you can twist them off easier with a breaker bar! LOL ----And when your 3/4 inch Sears socket splits you can have it replaced free!

Seriously , when you torque either grade down to 50-55 foot pounds, the friction it generates between the coupler and tongue will way exceed any forces trying to shear the bolt. In essence, the bolt will not have any shear forces on it if torqued correctly. I used 50-55 as the number because that is what I remember for grade 5. grade 8 can take a higher torque. AND----If you really want overkill, buy a couple of grade 8 1/2 X 20 fine thread. They take an even higher torque.

 

[dingbat]

Re: Replacing the actuator

+1

80,000psi, 130,000psi, The tongue only sees a few hundred pounds!

4 or 6 of ANYTHING will still be way overkill!

You do not have as much wiggle room as you think.

A 1/2" bolt has 0.126 sq. inches cross section in the threads. Divide your 130K pounds per square TS x 8. Your down to 16,000. Shear strength is typically 60% of TS so you are down to 9,600 pounds and we still have not addressed the fatigue strength of the bolts. Still need to look at the physics of #3500 of boat and trailer moving on the road at 60 MPH...... a #100 women suddenly becomes heavy when wearing high heels

 

[Frank Acampora]

Re: Replacing the actuator

But Dingbat---You are giving a shear example assuming the coupler and tongue will move freely across each other--like the receiver and drawbar. Not so. The clamping forces between the coupler and tongue are very significant. Try hammering a properly torqued coupler with a 12 pound sledge. You will destroy the coupler but not move it on the tongue.

And while I am talking about the receiver and drawbar, they only use a 5/8 pin. Six 1/2 inch bolts have a cross section of .756 square inches. The drawbar pin only has a cross section of .625 and that is in total shear every time you accelerate or stop. SO: Which will fail first?

 

[Bondo]

Re: Replacing the actuator

You do not have as much wiggle room as you think.

A 1/2" bolt has 0.126 sq. inches cross section in the threads. Divide your 130K pounds per square TS x 8. Your down to 16,000. Shear strength is typically 60% of TS so you are down to 9,600 pounds and we still have not addressed the fatigue strength of the bolts. Still need to look at the physics of #3500 of boat and trailer moving on the road at 60 MPH...... a #100 women suddenly becomes heavy when wearing high heels

Ayuh,.... Especially when shes walkin' down yer back,... wearin' Stilettos...

 

[bruceb58]

Re: Replacing the actuator

But Dingbat---You are giving a shear example assuming the coupler and tongue will move freely across each other--like the receiver and drawbar. Not so. The clamping forces between the coupler and tongue are very significant. Try hammering a properly torqued coupler with a 12 pound sledge. You will destroy the coupler but not move it on the tongue.

And while I am talking about the receiver and drawbar, they only use a 5/8 pin. Six 1/2 inch bolts have a cross section of .756 square inches. The drawbar pin only has a cross section of .625 and that is in total shear every time you accelerate or stop. SO: Which will fail first?

if the bolts are horizontally mounting the coupler, as most are, the downward force is likely the greatest component of shear. The sliding or stopping is contributing very little shear. The shear force that the drawbar pin sees is also small, especially with brakes...may only be 50 lbs or so during a stop. It's the impact of going over a bump that is contributing the most shear as the forces due to the impact creates the most force.

 

[UncleWillie]

Re: Replacing the actuator

You do not have as much wiggle room as you think.

A 1/2" bolt has 0.126 sq. inches cross section in the threads. Divide your 130K pounds per square TS x 8. Your down to 16,000. Shear strength is typically 60% of TS so you are down to 9,600 pounds and we still have not addressed the fatigue strength of the bolts. Still need to look at the physics of #3500 of boat and trailer moving on the road at 60 MPH...... a #100 women suddenly becomes heavy when wearing high heels

Good argument!

But 9600lbs x 4 = 38,400lbs
And as stated they are not loose pins taking the load in pure shear.
The clamping force transfers the load to the Coupler-Tongue interface.

A 3500lb trailer taking a hit that transfers 10g's into the frame will yield one G at the tongue with a 10:1 tongue to load ratio.
A 3500lb impact on our 38,000lb bolts allows for plenty of safety factor.
And a 10g impact would be a real Kidney buster!

Anecdotally, consider how many trailers are on the road that have been Red-Necked with grade #2 bolts and still work fine.

The part of the entire trailer system that amazes me is that skinny neck at the base of the ball.
If I didn't know better, I would think that would be the number one failure point.
Even in a collision, the ball and coupler always seem to remain intact.

 

[smokeonthewater]

Re: Replacing the actuator

well if ur gonna be all fancy with ur numbers don't forget to factor in the leverage ratio of the coupler IE distance between ball and bolts..

:nono:

 

[dingbat]

Re: Replacing the actuator

A #5000 boat and trailer combo traveling at 60 MPH has ~ 301 tons (#602,000) of kinetic energy. If that combo rear-ended a vehicle at 30 mph, the force of the initial impact would exceed 75 tons (#150,000)

The entire argument is a moot point anyhow. What are we talking here, a buck or two at best?

But Dingbat---You are giving a shear example assuming the coupler and tongue will move freely across each other--like the receiver and drawbar. Not so. The clamping forces between the coupler and tongue are very significant.

“Significant” is relative. The best you could hope for with grade 5 bolts is a clamping force of #9000. Figure in the coefficient of fraction of steel on steel and you end up with a connection in the #5000 – #7000 range. Insignificant if you consider that the connection could be subjected to 20 times that figure

And while I am talking about the receiver and drawbar, they only use a 5/8 pin. Six 1/2 inch bolts have a cross section of .756 square inches. The drawbar pin only has a cross section of .625 and that is in total shear every time you accelerate or stop. SO: Which will fail first?

The one with the greatest load as a percentage of it’s shear strength.....the bolts

The wall thickness of the receiver is 1/4”. The wall thickness of the actuator is 1/8”. If we where to apply #1,000 to each, the ?” thick receiver would transfer the load to the pin at 4000 psi (#1000/0.25) while the 1/8” actuator would transfer to the bolts at a rate of #8000 psi (#1000/0.12). You also have to figure in the fact that one connection is threaded and the other is solid. Makes a big difference when looking at fatigue strength of the connection

 

[Frank Acampora]

Re: Replacing the actuator

Ahhh! You want so desperately to be right!

Thickness of the parts doing the shearing (receiver and drawbar) has nothing to do with the shear force exerted. The shear is exerted at the junction between the parts, not at the faces where the pin is bearing, so your numbers have no meaning.

Granted, the receiver and drawbar are not designed as shear presses but they still will exert the full force of the shear on the pin. And granted, the shear resistance of the pin needs to be doubled because The drawbar is trying to shear both sides.

And clamping force on the coupler IS significant because other members will bend or fail first.

I do have to agree that in your example it is a moot point because at a 30 MPH collision with a stopped object, not only will the trailer tongue bend, but the winch post will most likely collapse and the safety chain would probably also break. The boat would end up in both vehicles. When shortening a vehicle that way, the LAST thing the vehicle occupants would be concerned with is the bolts holding the coupler on.

Last item: in your 30 MPH scenario, the first thing to fail would be the ball clamp mounting pin. It is only about 5/16 diameter

At any rate, Back to my original reply: I will continue to use short bolts on the coupler because it:

1. Looks better to me
2. It is easier to torque the bolts without having to worry about deforming the coupler and tongue.

 

[bob johnson]

Re: Replacing the actuator

will they ever experience the forces strong enough to shear them??? i highly doubt it.....any force applied would move the tow vehicle itself...even with brakes applied the vehicle would move before the bolts sheared......and if it got hit hard enough in a sudden crash....what happens to the bolts is a moot point......so in reality, you are fighting with twice as many bolts and nuts with horrible access to boot!!!

you dont need to torque the crap out of the long bolts, like someone said, youll crush the tube....three long bolts is over kill anyway....


good luck

bob