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#1
11-15-2004, 03:47 PM
 mceagle Gold Member Join Date: Oct 2003 Location: Broken Hill, Australia Posts: 1,188
Technical Question

Can one of you learned types answer this question for me.
If a prerotator pinion has 260 ft.lbs of torque and it has a ten to one reduction through the ring gear, there should be 2,600 ft.lbs torque at the centre of the rotor. What is the shearing torque on one of the two pillow block bolts 1" from the centre of rotation?
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Knowledge can be learned, but wisdom only comes with time.

Tim McClure
#2
11-15-2004, 04:21 PM
 Jerseywing Senior Member Join Date: Sep 2004 Location: North Arlington NJ Posts: 335

hmmm lets see...
carry the one,
damn i have to take off my shoes
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Michael Avenoso
PRA, EAA, AOPA
So much to learn, so little time
#3
11-15-2004, 05:58 PM
 Victor Duarte troublemaker Join Date: Aug 2004 Location: 43 35 14 N 02 44 46 E Posts: 1,714

Tim if you talk about the bolts holding the crown, it is a lever calculus, are you talking about that ? a drawing ?
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Victor Duarte
#4
11-16-2004, 12:27 AM
 mceagle Gold Member Join Date: Oct 2003 Location: Broken Hill, Australia Posts: 1,188

The bolts holding the pillow block to the hub bar - sometimes only two 1/4" bolts
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Knowledge can be learned, but wisdom only comes with time.

Tim McClure
#5
11-16-2004, 06:08 AM
 Victor Duarte troublemaker Join Date: Aug 2004 Location: 43 35 14 N 02 44 46 E Posts: 1,714

Tim, sorry i didnt take time to make a drawing (not at home)
the basic formula for a lever is : T1.d1 = T2.d2

T and d being torques and distances it is applied from the pivot..
so if you know the torque a the crown's teeth you know T1 and d1(crown radius)
D2 is the distance from the bolts to the pivot ( take a little more).
by filling the formula you can know T2 = (T1.d1)/d2, the torque transmitted to one bolt.
if you have 2 bolts , divide it by 2, and perhaps take a safety factor of 1.5..
someone should verify my statements .
thanks
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Victor Duarte
#6
11-16-2004, 08:35 AM
 Victor Duarte troublemaker Join Date: Aug 2004 Location: 43 35 14 N 02 44 46 E Posts: 1,714

a drawing
someone tell me if i'm correct ?
thanks
Attached Thumbnails

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Victor Duarte
#7
11-16-2004, 09:45 AM
 Doug Riley Platinum Member Join Date: Jan 2004 Posts: 5,389

I'm not learned, just barefoot at the moment.

2600x12=31,200 lb. at the one-inch radius.

If there are two bolts, then they each bear a load of 15,600 lb.

The X-sec. area of a 1/4" bolt is pi x R x R = 3.14 X .125 x .125 = .049 sq.in.

The theoretical shear load is 15,600/.049 = 318,000 psi. Ridiculous.

Obviously, that's not what's going on.

One thing that may hold things together is friction. Just as with a prop on a hub, a significant amount of drive force can bypass the bolts and be transmitted by friction between the rotor hub and the teeter block, if the bolts are snugged down. If they aren't, shear on the bolts is all you have.

Last edited by Doug Riley; 11-16-2004 at 09:48 AM.
#8
11-16-2004, 10:29 AM
 Victor Duarte troublemaker Join Date: Aug 2004 Location: 43 35 14 N 02 44 46 E Posts: 1,714

doug,
so there is no answer but : bolts strong enough to hold the parts together by friction.
380.000 psi is about 2700 Nm, 270 Kg/m doesnt sound so ridiculous...how do you perfom calculations ? with coeff of friction ?
thanks
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Victor Duarte
#9
11-16-2004, 10:35 AM
 Doug Riley Platinum Member Join Date: Jan 2004 Posts: 5,389

Victor, sorry to be a crude American (as usual for us lately!), but I have no feel for the metric system. AN bolts have a tensile strength of 125,000 psi. Shear strength will be equal or less.

Perhaps there's a difference in the use of the comma and decimal point?

"125,000" in our usage is "one hundred twenty-five thousand." Is 125,000 in European usage equal to just 125?
#10
11-16-2004, 10:49 AM
 gyropilot GyroBee N392JL Join Date: Oct 2003 Location: Shoreline, WA Posts: 733
Typo?

Doug,

Yes... outside of (primarily) North America and England, a comma is the equivalent of our period (and visa versa) in numbers. Can be confusing.

Regards,

John L.
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Last edited by gyropilot; 11-16-2004 at 10:53 AM.
#11
11-16-2004, 12:11 PM
 gyropilot GyroBee N392JL Join Date: Oct 2003 Location: Shoreline, WA Posts: 733
Shear Force

Doug,

In a rotor head prerotation application, I wouldn't think the maximum theoretical torque your calculated would ever be reached because:

1. The rotor isn't fixed and moves. Any shear force experienced on the teeter block retaining bolts would be very brief and would be just at the moment the rotor blades begin to move.

2. 100% of the maximum possible prerotator torque isn't typically applied when first starting the rotor blades moving. Usually the prerotator drive system is allowed to somehow slip until the torque is rather light.

If this were not true, we'd here reports of sheared bolts left and right. No?

Now maybe if the rotor blades were rigidly locked down and 100% available torque were "dumped" into the prerotator drive system, we'd see parts flying off.

John L.
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I would rather know a fearful truth than remain deceived by comforting falsehoods. - David P. Crews
#12
11-16-2004, 12:31 PM
 Udi Platinum Member Join Date: Oct 2003 Location: Beautiful CO Posts: 2,264

I find it hard to believe that a flexible shaft of the type we use in gyos can deliver 260 ft-lbs of torque...

Udi
#13
11-16-2004, 12:51 PM
 Al_Hammer Platinum Member Join Date: Oct 2003 Location: Madison, CT Posts: 1,261

John and Udi, those are both good points you raised
.
if the rotor is spun at 200 rpm and the reduction is 10:1 That means the rpm at the driving end is 2000 rpm.
Hp= (torque X rpm)/5252, with torque in ft-lbs. so hp = 260 X 2000/5252 = 99 hp.
Not likely that a flex shaft can deliver that kind of power.
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Al Hammer
#14
11-16-2004, 01:09 PM
 Doug Riley Platinum Member Join Date: Jan 2004 Posts: 5,389

...and, of course, with 99 hp to the rotor it would be a helicopter, not a gyro with prerotator.

People have tried large motors at the rotorhead that develop 8 hp or more. That puts us at 1/10 the values we started with; still too much for two 1/4" bolts. The engines used on small gyros usually don't have 8-10 hp to spare for prespinning, however, unless you can de-pitch the prop. With peaky Rotax 2-strokes, the spare power looks like only a couple of hp above what the prop absorbs.

John Landry -- I thought the comma/period thing might be the culprit. I'm just too lazy to convert metric areas to English to see for sure.

You might get a torque spike if you suddenly jammed on the prerotator with the blades stopped, if you had the engine throttled up at the time. The flywheel energy of the prop would be added to the engine's power output for a moment. With the Dominator hydraulic prerotator, if you suddenly squeeze the handle with the engine at /near idle, however, it will just kill the engine. This is true even of the Rotax 912S, with its flatter power curve.
#15
11-16-2004, 01:40 PM
 Victor Duarte troublemaker Join Date: Aug 2004 Location: 43 35 14 N 02 44 46 E Posts: 1,714

Quote:
 Originally Posted by Doug Riley Victor, sorry to be a crude American
No problem doug, i also have headaches when converting US to metric
so i use this converter (a good idea should be to have one on rotaryforum)
http://www.igus.com/conv.htm

ok al,udi and john the max torque is on acceleration only, so, how should Tim calculate his bolt shear?
thanks
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Victor Duarte

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