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Moment Calculation Help

Moment Calculation Help

(OP)
Greetings:

We have a table that holds a work piece and we wish to enable the operator to work on the table face up, or rotate it 90° so that the table is facing operator as they stand straight up. With that, the round table is held at (2) ends with a shaft and bearings, the distance between bearings is 56.50 in. The weight of the table is 1,000 lb. The shaft coming out of the gear box is .50" I need help calculating the whether the torque rating for the gear box is sufficient or not. Can someone help me with this application?

I have attached a diagram to help with the visual.

Thank you in advance for the assistance.

Brent

RE: Moment Calculation Help

Brent

I looked at your sketch. I dont think you have enough information to really solve. You need to know the eccentricity (distance from the shaft to center of mass of fixture plate and any other appurtances) to solve for the moment your gearbox needs to overcome.

If your 500lb gravity load line of action passes through the shaft or very near it, there really isn't a moment (its pretty balanced) and no real work on gearbox. I would also look at the structure required to hold the fixture from gravity loads. Ensure your shafts are properly supported and not in substantial bending. Don't put additional stress on the work piece

Jeff
Pipe Stress/Structural Engineer
www.xceed-eng.com

RE: Moment Calculation Help

(OP)
Ok, yes, the weight (1000 lb supported at two points (bearings) 56.50 in apart) are in line with the shaft that will rotate the fixture. I am trying to calculate what type of torque rating my gear box needs to have to be sure it is rated properly for the application. The gearbox will simply hold the plate horizontal to the floor, or it will rotate the fixture 90° and hold in 2nd position. The bearings will ultimately support the weight, but the gearbox will need to have some type of rating to be able to twist this weight to and from (2) positions.

RE: Moment Calculation Help

To get a reasonable torque value, assume that your successor has rotated your fixture by 1/4 turn in its own plane, and then folded it in half, so that all the mass is concentrated at the most extreme fixture plate radius. The weight of that mass times that radius gives you a moment for design purposes.

The next problem you will run into is that a common gearbox will allow enough backlash so that the fixture plate will wobble around the gearbox output axis as the fixture is used, annoying the operators or destroying the accuracy of any related stuff that doesn't move with the fixture. The backlash should be smaller with a worm gearbox, especially if the last pass has a numerically high ratio, but it won't be zero.
So, if you want the _fix_ture to stay _fix_ed wherever you put it, you need brakes on the fixture axis; a brakemotor won't do.
If you want the fixture to stay fixed at a finite number of positions, you need to add shotpins, basically tapered plugs that are pushed into tapered holes and held by a toggle or an air cylinder or something similar, positively locking the table.

Mike Halloran
Pembroke Pines, FL, USA

RE: Moment Calculation Help

BrittToolEngineer,

We need the maximum side (shear) load your table will exert on your shaft. We need the maximum torque your table will exert on your shaft. This torque is the maximum off-centre load your table will experience. This may occur when your tables is turned sideways and your work piece is cantilevered off the side. It may occur when a worker sits on one edge, or hammers at something on one edge. It may occur when somebody runs a forklift into this thing.

I am pretty sure you want a high ratio worm gear, so that it will retain itself in position without a brake.

--
JHG

RE: Moment Calculation Help

(OP)
Thank you all for the information.

Much appreciated.

RE: Moment Calculation Help

You're still missing the CG location of whatever is mounted on your fixturing table.

If the axis of the shaft which the table rotates around is x, you need the location of the CG of the complete setup (table + work piece + clamps + sensors + whatever else is mounted on the table) in Y and in Z.

When the table is parallel to the floor, there will be some moment applied to the shaft (Y x Mass). When the table is perpendicular to the floor, there will be another moment applied to the shaft (Z x Mass).

In any intermediate position the total moment will be some other value, which may be greater than the simple mass x Z or mass x Y values depending on where exactly the CG is in space.

You need all of this information to calculate gearbox torque.

RE: Moment Calculation Help

You also haven't defined the external loads that will be applied to the fixture plate, i.e.:
- Drilling
- Hammering
- Pressing

And you haven't defined how far you can allow the fixture plate to deflect under those external loads.

I'm reminded of an expensive custom fixture made to hold aluminum workpieces under a mill, in several orientations, so that the CNC mill could plunge, bore and thread the parts on four sides. The fixture was made of aluminum, and worked okay, except that under the axial pressure from a big drill, it deflected almost half an inch, which meant that the holes were drilled too shallow.
I suggested that the fixture needed to be redone in steel, with a few support bearings added.
I don't know if that actually happened, because the suggestion was not well received.

Even without working out the deflections, I wouldn't want a half ton fixture plate hanging from a 1 inch round bar, and what will probably turn out to be a couple of cheap bearings.
... but that's just me.




Mike Halloran
Pembroke Pines, FL, USA

RE: Moment Calculation Help

(OP)
How about we assume the maximum moment the gear box needs to overcome is 28,500 in-lb. The gear box chosen is model 730B-50-G. The ratings for this gear box are as follows:

710B-50-G
710 S/R Red. 50:1 Ratio, "B" Horiz. Base, "G" ASSY

Ratio: 50:1 Output RPM @1750: 35.0 Output Shaft: Projecting O/P Product Family: 700 Series Input Style: No Flange Reduction: Single Base Mounting Attachment: "B" Horiz. Base, Bottom Mount Fan Cooled: No Input Frame Size: None Max Input Power (HP): 0.150 (hp) Max Output Torque Cap: 153 (lb-in) Motor HP: 0.17 Output Shaft Projection: G-Left Output Shaft Style: Solid Shaft Assembly Code: G Pre-Lubricated: No Flange: No Recommended Lubrication: Klubersynth UH1 6-460


With using a crank on the Input shaft of the gearbox, would an operator be able to rotate this table with a low force?

Thanks,

Brent

RE: Moment Calculation Help

common sense says this fixture is going to distort, what is the weight of the plate and what is the thickness and diameter of plate. what material is it. looking at the 3/8 diameter will not be rigid enough. what is the purpose of this fixture, and what is the accuracy required. shorten the length to make it more rigid, increase the diameter of the shaft, whats the max flex required?

RE: Moment Calculation Help

Quote (BrittToolEngineer)

How about we assume the maximum moment the gear box needs to overcome is 28,500 in-lb. The gear box chosen is model 730B-50-G. The ratings for this gear box are as follows:

710B-50-G
710 S/R Red. 50:1 Ratio, "B" Horiz. Base, "G" ASSY

Ratio: 50:1 Output RPM @1750: 35.0 Output Shaft: Projecting O/P Product Family: 700 Series Input Style: No Flange Reduction: Single Base Mounting Attachment: "B" Horiz. Base, Bottom Mount Fan Cooled: No Input Frame Size: None Max Input Power (HP): 0.150 (hp) Max Output Torque Cap: 153 (lb-in) Motor HP: 0.17 Output Shaft Projection: G-Left Output Shaft Style: Solid Shaft Assembly Code: G Pre-Lubricated: No Flange: No Recommended Lubrication: Klubersynth UH1 6-460


With using a crank on the Input shaft of the gearbox, would an operator be able to rotate this table with a low force?

If 28,500 in-lb is your actual service load, you have specified a very small gearbox where a very large one is required.

28,500 in-lb applied to a 3/8" shaft results in a torsional stress of approximately 2,752,000 psi... you need a much, much bigger shaft just to handle rotating the table from position A to position B.

Once you add in other loads as MikeHalloran has (very prudently) suggested, you need to account for them. For the fixture to be stable and not move under all of this loading, and for it to be safe for an operator to work in/around/in front of, you also need a relatively large safety factor.

Again, if 28,500 in-lb is your actual service load you need BIG gearbox. That's almost 2400 ft-lb.

For reference, I flipped through the 700 series catalog from Boston Gear. The largest 90 degree reduction unit they make off-the-shelf is rated for only (only!) 19,000 in-lb, has a 600:1 reduction ratio- meaning your operator will need 150 turns of the hand crank to move the fixture 90 degrees (ouch if you care about ergonomics at all). It also weighs a couple hundred lbs and has a 2.5" output shaft.

RE: Moment Calculation Help

There's another option that takes care of the gearbox torque rating problem, reduces the angular backlash from the gearbox, and can save quite a bit of money; roller chain.

Attach a humongous flame-cut roller chain sprocket to the fixture plate rotation axis, and drive it with a much smaller roller chain sprocket on a modest gearbox attached to the floor, or whatever structure holds the fixture plate up.

DO NOT FORGET to include a full coverage guard over the entire roller chain assembly to keep stupid hands out of the mechanism, with an easy opening door so the chain can be lubricated.
DO NOT FORGET an adjustable tensioner and idler sprocket within the guard so you can keep the chain tight.

Mike Halloran
Pembroke Pines, FL, USA

RE: Moment Calculation Help

Since you only want to rotate 90* a shaft and gearbox is probably not the most efficient solution. Can you attach a lever to the table and move the end of that lever with a screw thread and handle?

je suis charlie

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