Load Factors For Rotating & Tilting Fixture Designs 2

[tc7]

The background: We just acquired an old, old mechanical positioner that we can use to support, tilt and rotate some very large forgings (valve bodies, gear housings, sheave housings) that we repair by welding. Some of these forgings are oddly shaped and can weigh up to 4000 lbs. The manufacturer of the positioner is unknown.

To attach these oddly shaped forgings to the positioner table, I will have to weld up some specialized fixturing. The forgings will be cantilevered off of the positioner plate while the positioner plate is held vertical. The positioner will be rotated and tilted in any attitude necessary to reach the spots in and around the forging in need of repair.

The problem: Although the tilt and rotate motor functions of the positioner move slowly, they do not have a soft start feature so I am concerned that fixture and attachment design must include inertial loads or a dynamic load factor to account for both the tilt and rotating starts & stops.

The question: Can anyone advise on how best to determine an appropriate load factor for this old positioner to be applied to the design of bolting and welds for the fixtures ?

The next question: Are their any commercial Codes that govern the best practice and SF’s of fixture design?

Thanks

 

[Twoballcane]

Can you get some accelerometers on it and measure the G loads when it start and stop? If you can not get any measurements or data, you're really guessing



Tobalcane
"If you avoid failure, you also avoid success."

 

[MikeHalloran]

Most of the fixtures that I have seen are strong enough to withstand machining loads that far exceed any inertia load on the workpiece. That includes fixtures for welding.

Consider this as an informal rule: If your fixture deforms noticeably or measurably the first time it gets hit by a forklift, you'll be a laughingstock on the production floor.



Mike Halloran
Pembroke Pines, FL, USA

 

[LSPSCAT]

I agree with Mike, typically large fixtures are governed by "when in doubt build it hell for stout". Deflections typically rule when it comes to fixturing anything for machining likewise for fixturing and welding large fabrications. Deflections and tolerances on fixtures are typically 1/2 of that allowed on the final assembly.

Stress wise for fixtures around the shop/construction site I sometimes design to ASME B30.20 for Below the Hook Lifting Devices. It seems conservative enough and has factored in 100% impact loads. Logical that most construction equipment takes as much abuse as any lifting beam or lifting lug connection.

 

[EngineerTex]

As a SWAG, I would take the rated power of the motor and then divide it by the nameplate speed * the gear reduction ratio. This should be an optimistically maximum possible torque, since an electric motor generally will not have a stall torque higher than its running torque.

Now that you know your max possible startup torque, you would then need to determine how much stopping torque would be induced by a sudden deceleration. For that, find the the maximum angular momentum you would have with a 4,000 lb tubular member of the maximum OD and length that the fixture could handle, moving at maximum speed. Now it gets a little tricky. You state that it has a hard stop -- how does that work? Does it have brakes? Does it have a worm drive? Does it have a large enough gear reduction that simply stopping the motor has enough friction to keep it from moving? At this point, you have to analyze the system to determine what type of spring rate you have between the inertial load and the stopping force. If you apply a few hundred foot-pounds of torque with a torque wrench to the rotating table, you should be able to measure the angular deflection to determine your angular spring rate between the rotating part and the part that does the stopping. Armed with the rotational inertia and the angular spring rate, you should be able to determine what type of torque load your welded fixture would see.

As for safety factors, I would disagree with LPSCAT on selecting the Below the Hook Lifting Devices. The reason is that it sounds like you're dealing with something that is probably suspending a load over parts of a person's body while in service. Because of that, I would beef up the safety factor from there (and I'm a guy who doesn't like ridiculously high safety factors). I would look at a code for elevators or man-lifts as a start to determine appropriate safety factors. As another SWAG, I would guess that you will likely end up with a 3:1 SF to start and an additional 2:1 SF for dynamic loading, to give a total SF of somewhere in the neighborhood of 6:1 for the overall design.

If, by chance, you do decide to look at an elevator code, don't get confused by the 10:1 SF for the wire rope. That's only for the wire rope. The structural safety factors are lower and more reasonable.

All that being said, MikeHalloran is correct in saying that if they run a forklift into it or drop the item from a couple of feet onto your fixture, it had better hold up. In that case, the design factor is moot compared to the abuse factor. It's the same principle with the bumper on your car -- normal duty, it's pushing air. But the actual design is based on the rare (?) cases of abuse.

Engineering is not the science behind building. It is the science behind not building.

 

[tc7]

LSPSCAT-
Thanks for the reference to ASME B30.20; There is a related publication, ASME BTH-1 which is a design handbook for below the hook devices. I am ordering both ASME docs today.

EngineerTex-
Thanks for the ideas on the startup torque and torque loads. Good stuff! and very practical that I will study up on over the next weekend.

Regards.