Arbor Press Torque Calculations

[killforfood]

I plan on using a click stop torque wrench to press rockets and comets on my new arbor press. I don't have a pressure cell so I needed to do the math to arrive at the proper torque settings. I started with an arbitry 1,000 lbs just to keep it simple and get my mind wrapped around the concept.

 

http://img.photobucket.com/albums/v236/killforfood/Pyro/TOOLS009.jpg

 

Problem:

 

Calculate the amount of torque needed to cause 1,000 Lbs of downward force with an Arbor Press

 

Solution:

 

multiply 1,000 by the gear radius of the pinion shaft. I’ll use 1” to keep the math simple. It’s actually slightly less.

This gives us an answer of 1,000 inch pounds of torque. Divide this answer by 12 to convert it to Foot Pounds.

 

Answer:

 

83 Foot Pounds of Torque.

 

 

Now I’m going to work up some numbers that I can use, as in SIMPLE so even I can understand it.

 

1. The surface area of a rocket is its radius squared times Pi (Pi = 3.14)).

2. For a ½” rocket, multiply .250r X .250r X Pi 3.14 = .196 Square inches.

3. Pressure needed to press BP rockets is 6500psi.

4. 6500 psi x .196 = 1276 psi.

5. The pinion gear on my Arbor Press has about a .750” radius.

6. 1276 psi times the .750” pinion gear radius = 957 inch pounds.

7. 957 inch pounds divided by 12 = 80 foot pounds.

8. For the truly anal, subtract the spindle area from the equation.

 

a. Side note: The actual contact point of the gear is what defines the gears true radius. It’s likely that the radius is smaller than my estimate and this would lead to greater torque multiplication (less foot pounds required).

b. I did the math for a 1 pound rocket and came up with 179 foot pounds. I don’t think my little 1 ton press could take much of that.

 

I would appreciate it if someone could verify my work.

 

Thanks, KFF.

[marks265]

Hey Killer,

 

I have a:

 

http://www.northerntool.com/shop/tools/product_20674_20674

 

I couldn't do very well with it for a 1 pound rocket with good fuel and universal tooling. That was a few years ago. I would have to revisit this and use my force gauge to see where I was at. I even tried drilling and tapping the pinion and that felt really ugly when cranking on it. Then I found myself having a heck of problem trying to secure the thing when pressing. That's when I quite and went hydraulic. Maybe I will see how much force I can put on a composition this weekend.

 

Mark

[coogan1997]

How does one attach a torque wrench to an arbor press?

[Col]

Easy option is to drill and tap the left side of the pinion for a high tensile bolt, use a socket that fits the bolt head on the torque wrench. Personally i`d go for a P-to-F guage which you can use on any press.

[DavidF]

KFF, I just checked mine. 1000 pounds on my Wolter PtoF gauge worked out to 86 pounds on my torque wrench. That's not to say your calculations are not perfect- I don't know. But, if you pull faster on the torque wrench, it clicks sooner than if you were to pull slow. So, 86 is the number that worked out to for my pull speed. Pretty darned close to your calculations!

 

I like to use the PtoF to set the torque wrench only. Pressing while watching a gauge is quite a bit more work on myself and my gauge. Just my druthers is all.

 

Some handy numbers: 1500 pounds of force on my gauge is 118 foot pounds on the torque wrench. To press a 1/2" whistle rocket to 8800psi works out to 127 FP on my gauge, which is 1724 on the PtoF. To press a 5/8" ID BP rocket to 6500psi is 190 FP, which is 1995 pounds on the PtoF, and is using the press at full rated capacity.

[mechsawyer]

How did you manage to attach a breaker bar to your arbor press? I am looking to add a torque wrench to mine and I'm clueless where to start. Thanks!

[OldMarine]

Grind flats on the the shaft so a socket will fit the end and stick it on with an epoxy cement. You can then use a standard torque wrench and remove it for ease in travelling.

[dagabu]

I just welded a socket to the end of the pinion gear shaft so that I can still use the rod if I like. The lesson I took away was to position the socket carefully before welding it on if using a rigid torque wrench.

[lloyd]

Did you also use a longer handle-holding bolt, so you can still tighten the rod handle, if you wish to? That's what we did on a couple of presses.

 

Lloyd

[dagabu]

No, I'm one of those guys that just end caps the handle or removes it completely until I need it. I really like a production line kind of component building, if I'm pressing one rocket, why not press 100?

 

Looking back at an old post, I see that I broke one socket off the end of the pinion and cut the socket back a lot to reduce the down pressure on the weld.

Edited July 13, 2017 by dagabu

[cevmarauder]

Not to be a thread necromancer, but I knocked out a spreadsheet that calculates torque values for rammers between 1/8 and 2" (in 1/8" increments) for any PSI value between 500 and 10000. Although, good luck finding a cheater pipe that can ram 10000psi onto a 2" tube.

All you need to do is plug the diameter (not radius) of your pinion gear in, and it'll do the rest. I uploaded it to the site, too--get it here: https://www.amateurpyro.com/forums/files/file/120-arbor-press-torque-calculator/

Let me know what y'all think, any ideas for modifications, etc. Hopefully I didn't screw up the formula...I tested it with killforfood's 1.5" diameter pinion example and 6500lbs came to 79.7, so I think we're good.

https://www.amateurpyro.com/forums/files/file/120-%7B%3F%7D/

Edited April 15, 2018 by cevmarauder

[lloyd]

Were you asking for the outside diameter of the pinion, or 'equivalent pitch diameter'? It makes a substantial difference with large-tooth gears like that! The o.d. of a gear is NEVER it's 'acting diameter'

 

<grin>

LLoyd

[cevmarauder]

Were you asking for the outside diameter of the pinion, or 'equivalent pitch diameter'? It makes a substantial difference with large-tooth gears like that! The o.d. of a gear is NEVER it's 'acting diameter'

 

<grin>

LLoyd

That's a good question! I'm a network engineer, not a mechanical engineer (and since I got snookered into management, not so great at that, even. I'm, uh, good at getting other people to do the real work?) How would we go about calculating acting diameter...relatively easily?

 

Using the o.d. would at least err on the side of caution, although the error grows with the diameter of the rammer.

[lloyd]

IF the gears are a 'standard' style, there's a little 'hump' in each tooth, where the nose joins the body of the tooth. The peak of that hump marks the (nearly exact) equivalent pitch diameter, since that's the part of the tooth that delivers the initial impulse to the mating gear or rack. With large-toothed gears like those usually are, it can make a significant difference between the o.d. and the equivalent diameter.

 

I must say, I'm impressed that you cared. Good on you! Most folks would have shrugged that off as a snide comment on my part -- and it was not intended to be.

 

Lloyd

[cevmarauder]

Oh, I never assume "snide" unless it's laced with profanities or sarcasm. I'd actually edited the original post and promptly forgot to save changes before wandering off to fix a chainsaw (I wonder where my kids get their ADHD from?)

ANYWAY, what I'd meant to say was, I did some digging, and after nearly suffering a seizure from the kind of math I scrupulously avoided in college, I came to the conclusion that if I ask for the O.D., and the number of teeth, I can figure out the Module by dividing the OD/(teeth+2), and then take the Module, multiply it by the number of teeth, and that should give me a fairly close shot to the reference diameter? In the case of mine, .875/(12+2), times 12, comes up to .750==3/4", which is pretty close when I bust out my ruler. If they've got honkin' huge gears, they're going to be standing across the room pulling their cheater pipe, anyway.

In other words:

(Outside Diameter/(Teeth+2)) x Teeth = Reference Diameter

Think that would be "Good enough for government work"? This of course assumes standard rack-and-pinion teeth.

Jeez, now I know how my wife feels when I start babbling about BGP route metrics and MPLS labeling.

[lloyd]

Yep, that would do. You initially asked for 'easy', so I didn't include the math. I should have... I was a non-degreed-but-working EE for most of my working life. I was a HAM by age 15, doing radio work in the Navy, and designing computers by the time I was 22. But my degree was in business management! Go figure!

 

Now (heh!) I run a small machine shop and design works for pyro manufacturing machinery. I have 'career ADHD' <smile>.

 

There was another question, asked elsewhere, that might help some folks here. It was mentioned that Wolter recommends 8800psi (on the composition) for whistle rockets, and Ned recommends 7500psi. "Which is right(better, whatever)?"

 

Presuming you have good-enough quality of tubes and sufficient tube wall support in your mold, the higher pressure will deliver more consistency of burn, at the cost of a little burn rate (not much). But since the minimum for reliable whistles lays down in the 5Kpsi area, either is plenty for good effect.

 

I designed 'non propellant' whistles for a military project, and we pressed them to about 3400psi with perfect reliability. Now... they were 'end burners', and designed for noise, not rocketry. But even they would fly as simple 'bottle rockets'. That low pressure wouldn't work for core-burners.

 

Lloyd

[cevmarauder]

Alright, uploaded an updated version that uses the simplified Module calculation. (It assumes a standard gear tooth height to diameter ratio--you've got some funky gear type, you're SOL. Just pull harder--more is always better, right?) Edited the original post to contain the newer version.

Thanks for everything, Lloyd!

[tenneyguy]

I have used my bench vise to squeeze stuff. Using a screw thread to achieve compressive force allows torque applied to determine reasonably accurately the force obtained.

 

The formula for threaded fastener tension using steel threads is : T = 0.2 F D, where T = torque, F = tensile force, D = screw

diameter. Thus, a 1/2 inch nominal diameter bolt torqued to 100 ft-lbs. will experience a tensile load of 12,000 lbs.

 

Bear in mind, units must be consistent: 100 ft-lbs = 1200 inch-lbs.

 

BTW, that torque on a 1/2" bolt requires Grade 8, or it'll BUST!