Rotating Lower Hook Block

[jdkuhndog]

Greetings.

I have recently completed the calculations to determine the required Torque for a gearmotor to rotate a hook (and its 6000# load) on a lower hook block for a crane.

My question is how do I determine if the lower block is going to rotate/spin instead of the load rotating?

Here is how I went about determining the required Torque for the gearmotor on the lower block for rotating the load:

T1 = Torsion due to load inertia
T2 = Torsion due to friction = 149 lb*in
WK^2 = mass moment of inertia of load = 3063 lb*ft^2
t = time to accelerate = 5 seconds
R = ratio between hook and gearmotor = 6.7
N = RPM of hook and load = 2.0


T1 = ((WK^2)* N)/(308 * t)
T1 = (3063 * 2.0)/(308 * 5) = 3.97 lb*ft = 47.7 lb*in

T3 = Torsion required at the hook to rotate = T1 + T2

T3 = 47.7 + 149 = 196.7 lb*in

Tmotor = T3/R = 196.7/6.7 = 29.4 lb*in

I could have reflected the WK^2 back to the gearmotor and then used the RPM of the gearmotor - and I am assuming I would have gotten the same result.

Anyhow, I have been debating what to do with the actual required Torque at the gearmotor in regards to determining if it will spin the lower block instead of the load.

Since it is a Torque from the motor does it matter the distance this motor is from the centerline of the lower block? Do I translate the torque from the pinion into a force exerted onto the ringgear (shank of hook goes through the ring gear) and multiply that force by the distance to the center of the hook block to determine the moment about the lower block?? Then what do I do with this moment? How does the WK^2 of the lower block resist the moment/torsion from the motor?


Thanks for any help you can give.

 

[alexit]

I believe you are considering the cable/chain for the hoist as a rigid structure in your above calcs. You need to consider these members as elastic.

When attempting to accelerate the load, there is a vertical component generated as the upper block rotates before the lower block follows. This component will change as the lenght between blocks increases. If your vertical acceleration component is greater than the acceleration of gravity, the upper block will spin and the load will "jump" before rotating.

Alex

 

[jdkuhndog]

The gearmotor for rotating the hook is located on the lower block. Let me try to be a little more clear on what is going on.

I have a lower block with 4 sheaves - 8 parts of wire rope...right now I am considering the wire rope to have no effect on resisting the twisting effect.

The lower block consists of a hook with a shank which goes thru (and is keyed to) a ring gear, then thru a bushing in a crosshead and then to a hook nut which will rotate on the bushing. The sheaves are above the crosshead and held in place with a sheave pin. The gearmotor for rotating the hook is mounted on the lowerblock off to the side with a pinion which meshes with the ring gear. The ratio between the ring gear and pinion is 6.7.

The design is for only the hook (and its load) to rotate.

My concern is if the mass moment of inertia of the load is so great that instead of the load rotating...the rest of the lower block rotates about the load and starts to twist the wire ropes.

The upper block is not connected with the rotation of the lower block in any way except the fleet angle that it imparts on the lower block which may help to resist some twisting by putting a horizontal load on the sheaves in the lower block. Since this fleet angle varies with the height of the lower block I have decided to ignore this in my calc.

I hope this is a little clearer now. (although it may now be even more confusing and I apologize for that !)

Thanks Again for any help !

 

[alexit]

This is clearer now. You must calculate the fleet angle and the effect of the sheaves on the lower block.

If you consider a system where you can suspend the lower block and load with only a vertical component the force (torque) loop exists only between the load and the lower block...if the inertia of the load is greater than that of the lower block the lower block will rotate not the load.

Since you have the 4 sheaves restraining the lower block, these must be considered to resist the lower block rotation.

Alex

 

[Ralph2]

Off hand I would suspect you are going to need something to isolate the torque to the sheave. Another set of bearings ?? so that the action can happen independantly. If you "fix" the free swiviling (sp) by a gear mechanism then surely the reaction will wind up the sheave and again when it your rotating force (motor) stops.
Interesting project.. my thoughts..for what they are worth..
Good luck
Ralph

 

[unclesyd]

I was considering the same thing mentioned by Ralph2 about where the reaction force would be.
The I ran across this item and having used many a hydraulic wrench I have more questions.

http://www.hytorc.com/html/xxi.html

 

[jdkuhndog]

Thanks for the comments...

I have made a phone call to a company which manufactures these lower blocks with a rotating hook.

The chief engineer there told me that they really don't have a calc that they perform to ensure that the wires won't twist...they have had it only happen to them once over their history of manufacturing these lower blocks.

He indicated that as long as the sheave spacing wasn't too narrow, then it should be stable enough to resist the twisting. (A little vague, huh?)


I guess I will just wait and see. (I will prob. play around with the horiz. forces created from the fleet angle of the wire rope and see what I come up with there)

Thanks again !

 

[minerk]

unclesyd,

Wow, that really is a lousy description of that hytorc product. I suspect that it really is a bolt tensioner (stretcher) and not a torque device. I'd like more info about it, but there is none to be had on that site!

 

[unclesyd]

minerk,

The link works.
Here is the tag line for the XXI.

The FIRST Hydraulic Torque Wrench without a Reaction Arm!
The FIRST Continuous Rotation Hydraulic Torque Wrench!

If the link doesn't work by clicking on it, either type it in or hold down the control key and then click on it.

unclesyd

 

[minerk]

The link worked fine. I was commenting about the quality of the site itself. There doesn't seem to be much useful information about what this thing is, what it's capable of, or how it works. They call it a "Torque wrench" but then go on to say it provides "Torsion-free Bolt Stretching" and "stop...all galling by not turning on the bolt." I have a hard time believing that this thing is a "torque wrench" based on the features they proclaim. It seems much more likely that this is a bolt stretching device. Hytorc needs to educate whoever is doing the marketing on this thing, because that website and the brochure it links to are horrible. Wouldn't a torque wrench without a reaction arm violate Newton's 3rd law? And doesn't a torque wrench provide torsion by definition?!?

 

[alexit]

I agree, the literature is useless...must be a translation.

Alex

 

[ccw]

Hi Jdkuhndog,
At the risk of getting back on track to your original query, first, the 147 in-lbs friction for T2 seems a little high for a good anti-friction thrust bearing.

The other day we rotated a 40,000 lb, 4 ft. dia x 20 ft. tall shielded shipping cask on a turntable bearing by hand. Working the numbers it seems your 147 is about twice as high as it could be.

Second, your first cut computations should not be concerned with T2 or rotator internal mechanics. You should only be concerned with the line angle off plumb influence (sin or tan (line angle off plumb) * load) x (distance from sheave to hook center) = torque from rotating the load or = rotational inertia of the load referenced to hook center line x angular acceleration/deceleration in azimuth of the load.

Consider the rotator system a black box between hook and traveling block attached. The traveling block is going to rotate in azimuth a small amount (wind up) by definition depending on how fast the hook accelerates in azimuth in relation to the traveling block (equal and opposite reaction). As someone pointed out, the greater the distance between the traveling block and the hoist drum, the more flexible the system will become, due to larger block rotation required to achieve the same angle off plumb for the hoist lines.

You stated that the rotator takes 5 seconds to go from zero up to 2 RPM. This defines an average angular acceleration. I assume there is some sort of ramp up control to give the 5 second duration. Does this imply there is also a ramp down control (equally important)? Once up to speed, and the wind up oscillation from initial acceleration has decayed to near zero, steady state is achieved. Quantitative predictions of the dynamic amplitude, period, and decay of the wind up involve many factors, such as stiffness of the boom or bridge; damping values of cable twist; tare load of traveling block, rotator, and hook assembly on cables**; distance of block from hoist drum; included angle between hoist line parts; any kind of environmental forces on the load itself; etc. Of course, this wind-up action will occur again when you stop the load rotation.

The worst that could happen would be if the load azimuth spotting control (man in the loop?) begins to "hunt" the sweet spot, over-compensates, develops a pole in the left half plane of the nyquist stability plot, and turns the whole thing into a giant yo-yo! It makes you appreciate the value of one or two professional riggers and tag lines in spotting loads.

In reality it seems that your hoist system load rating is way overkill for a flimsy 3 ton load, and if you do your load rotation v-e-r-y slowly and have some reasonable payout (< 100ft), everything should be OK. (maybe go for 1 rpm max. rather than 2 rpm).

** You can minimize dynamic wind-up by attaching a heavy and/or large rotational inertia object to the traveling block frame, whose weight plus the load weight brings you close to the safe working load of the hoist. (This is the same reason the earth does not wobble very far or fast when I jump up and down on it.)

 

[sacem1]

I would see your problem the other way around, not from a twisting force but from a lifting force, I'll clarify:

When the ropes (8 in this case) twist they slightly change in lenght efectively raising the cargo and that implies that the twisting force is greater than the weight of the cargo, not only the tension of the ropes/cables (1/8th of the cargo weight) so that usually is enough to avoid the twisting of the ropes.

If you have the problem because the acelaration of the rotating cargo is to high you can increase the lifting of the cargo weight by increasing your block sheaves diameter and in doing increasing the cargo lifting effect of the rope/cable twisting.

Thats the reason you can rotate a cargo on a crane and never twist the ropes unless the block spindle bearing is stuck.

SACEM1

 

[jdkuhndog]

ccw,

don't have much time to write...but concerning one of your first points about the Torsion due to friction being too high...this is how I arrived at the number...let me know if there is a better way.

The nut is rotating on a bushing not on an anti-friction bearing. The coefficient of friction between the nut and the bushing is 0.03. (f=0.03)

I wasn't real sure how to translate the coefficient of friction of a surface that was rotating into a torsional equivalent...so I remembered a calc. that I had done a year or so ago that required torque capacity of a clutch...

From Mark's Handbook 9th edition, page 8-39

T=0.5*i*f*F*D

i = number of pairs of contact surfaces = 1
f = coefficient of friction = 0.03
F = axial Force applied = 6100#
D = mean diameter = 1.625"

T = 149 lb in


Thanks for the other input as well...I will try to read through it and respond as I get some time.

jdkuhndog