Does anybody know a formula to figure out the minimal clamping force needed for a spinning prop based on the pitch, the diameter, the max PRM, and the mass of the prop? Given a clamping force, one can calculate the minimal bolt torque.

No,
You do however need a different dataset in order to calculate the required clamping force.

Engine torque data curve during a complete power cycle.
engine rpm and rpm (angle velocity) fluctuations during a cycle. (engine/prop  inertia)
prop inertia and mass
prop aerodynamic torque
prop to engine friction coefficient
prop to engine mean friction surface diameter.
safety factor

There is another group of factors. maximum permissable bolt torque before failure.
Bolt material and grade.
Material the bolt is threaded into.
Compressibility of the clamped component.

At least one company figures bolt torque using only the shear values of the bolt. Then they add a few inch pounds. No consideration is given to the threaded side, though it is much softer than the bolt. No consideration is provided for the density of the propeller hub. Wood is treated the same ad a more dense center like fiberglass. This is why theyu had serious issues with one of their larger engines. people were seriously crushing wood props using the manufacturer provided torque values.

Even now they advise 75 to 85 inch pounds for an m5 screw, with no comments about the grade of the screw. Before they were advising over 100 in lbs.

The standard for metric socket head cap screws is grade 12.9. Does anyone use anything else?

YOU ARE RIGHT USING A TORQUE WRENCH BUT THERE IS ANOTHER STEP YOU DID NOT MENTION WHEN INSTALLING A WOOD OR COMPOSITE PROP. THE MATERIAL COMPRESSES UNDER THE TORQUE LOAD BUT DOES NOT STOP COMPRESSING . YOU SHOULD WAIT FOR THE MATERIAL TO COMPRESS AND THEN RETORQUE AFTER A A PERIOD OF SEVERAL HOURS OR LONGER. THE TORQUE VALUES WILL CHANGE CONSIDERABLY WHEN YOU RETORQUE.

[link]http://www.futek.com/boltcalc.aspx[/link]
An interesting calculator for bolt torque and tension.

How about the bolt strength of a single 3/8-24 grade 8 Lake Erie brand bolt ? You could tow your car with one...NO problem holding a wimpy little prop on a hub....
I will always believe that a single 3/8 bolt will hold ANY size prop, ANY material, if used in conjunction with a PROPER size hub and washer,(covers the WHOLE width of the prop) will work just fine, and ALL my race engines up to 30 HP are set up that way...We have not lost a prop in the last 18 years of racing at speeds up to 240 mph...Our latest 80 is turning 12,000 using a SINGLE 10mm grade 12.9 prob bolt and a carbon fiber prop..We use TIGHT instead of a torque wrench...

Each properly tightened, does either the muliple bolt hub or the single bolt hub offer any distinct advantage over the other? What is the optimum number of bolts for a hub? If the answer to the second question is one (1), that answers the first question!

Regards,

Richard

i plan on buying a 1/4 torque wrench from harbor freight.... would they sell the socket that has the allen wrench socket fitting that would torque my prop bolts?

Ralph,

The old arguement between a single center bolt and a multi bolt hub is a non sequiter here. It doesn't matter because none of the manufacturers will revert to a single bolt application.

However, for the sake of arguement, how does one compensate for the severe hub compression experienced when using a wood propeller and a single center bolt? Previous comments about using large wrenches to get the center bolt or nut tight enough to prevent slippage do not provide for the prevention of severe hub compression, which most certainly does happen. A correclty torqued propeller using a multi-bolt hub exhibits virtually zero hub compression. One over torqued propeller will indent both sides of the prop with the aft face usually more compressed than the front.

Happy birthday again btw.

I made mine from a 5mm hex key, J-Bweld epoxied into a short 5mm 1/4" socket from a cheapie ratchet set. Then I did a 6mm, and a 5/32. Usable hex keys for getting cutoff and glued are $1 at WalMart. They also have cheapie sockets and JBWeld. Easy.

However, a proper washer on the face of the prop will distribute the clamping force of a single bolt over the entire prop hub, thereby equaling the distributed compressive forces a multibolt hub provides. And, the holes drilled in the prop and the hub for multiple bolts have to somewhat compromise the strength of both.

Also, are not some of the engine manufacturers reverting to single bolt hubs instead of multiple bolts?

I am not commenting in the interest of arguing, but in the interest of enlightenment. Thank you for your inputs.

Sincerely,

Richard

????
Where did that come from?????
For your information (read slowly and carefully):
Bolt qualities range from 4.6 to 12.9 . Standard socket heads are in the better qualities only, and have 8.8, automotive grade is 10.9, special grades have 12.9. Most prop bolts are 8.8!
The first number denotes ultimatebreaking tensile strengthlimit/10, the second number is a multiplier for wherethe material just starts to flow (Hooke's law). Hence the number never can be 10, or the material would have no stretch at all, but only strain elongation within Hooke's law (0.2% strain limit, not resulting in permanent stretch). It would snap without warning.
example:
8.8 has 80N/mm2 ultimate tensile strength and 8x8=64 N/mm2 Hooke's law tensile strength (the safe limit, before serious deformation starts) That is why you should use screws, that are torqued beyond the stretch limit, only once, like in car clutch plates.

Those reverting to single security are only doing so on "little" gassers. Not many 50cc and up going that route.

You are correct in stating the center nut provides equal force over the face of a prop washer, but no more or less so than a mulit bolt. With a multi bolt those forces are moved outward a small amount. Better-worse? Don't know. In either type over torquing really screws up a prop. Pull most any wood prop at your local field and look at how deep the reaf hub indentations are. You will find most props are grossely over torqued unless a torque wrench was used.

TOM, You address the issue correctly. Only friction can hold the prop in place. Friction is clamping force times friction coefficient. (note that clamping surface does not enter the picture here)
In order for the prop to resist the required clamping force, it must have sufficient compression strength (enter clamping surface here)
Hard materials like carbon reinforced plastics have a higher compression resistance than wood. So wood needs large prop washers. If it compresses with the required clamping force, just use larger washers. That is all there is to it.
No locating pins, just the required surface. That should end the single vs multiple bolt discussion. In both cases, the required surfaceto prevent propshiftis the same!
To humour Ralph on his birthday: (here's to you Ralph!)
When the chosen solution is inadequate (engineering error!), the single bolt construction is safer, save for overcompressed hubs like in the original picture of TOM, where the prop parts company with the motor anyway.

Hey,
Have you not noticed that the world went metric while you were sleeping?
Just teasing.
Good values for Imerial sizes. How about metrics?

I'm with you on the grade thing Pe. How did 12.9 suddenly become the "standard"?

The better manufacturers provide an 8.8 screw. There are no grade markings on hardware coming from China so nobody knows what grade they are. A 12.9 screw supplied with a new engine would be quite an unusual thing to see.

sounds very complicated with this set.

Manufacture of props for full-scale airplane do publish prop bolt torque specs. How do they come with the numbers?

Yes, I made a big error. I was looking at Machinery's Handbook and quickly found the line "Alloy steel socket cap screws conform to ASTM574M property class 12.9....." I missed the line that said "Steel (hardware) conforms to SAE1199 or ASTM F568." A quick look at what is actually available for purchase would have made it obvious. Even so, Mcmaster Carr lists only two grades of M5 socket head cap screws, 12.9 and "unrated" stainless steel.

Pe, if you look just under the title of the bolt calculator you have a selection for metric, but material doesn't follow the metric grading system.

I took a few minutes to look up full scale practice. First, they don't seem to make many composite one piece props. Wood props are either tightened to a torque value or crush dimension (.006 inch per inch of hub thickness per Sensenich). Torque values specified for the bolts are stand torque values for AN hardware. But, standard torques for AN hardware is ~30% of ultimate stress. AN6 torqued to 200InLb as recommended by a [link=http://photos.imageevent.com/qdf_files/enginefiles/Installing%20Wood%20Propellers.rtf.pdf]Sensenich document[/link]. However, we all know that much time and effort was spent in engineering the proper clamping force and are often only certified for one type of engine.

Pe, the MVVS 50 shows 6 M5 screws on your technical drawing on your page. By torquing to a suggested 5.8 Nm (51 in-lb) for grade 8.8 in the chart you posted, this is 1300 Lbs of force PER SCREW! Pleas excuse the mixture of units. Times 6 bolts is 7800 Lbs! Do we really needs this much clamping force? A DLE 55 has 4 M5 screws, 5200Lbs. What does the prop manufacturer have to say? I would guess a single 3/8" nut would be 5000Lbs maximum.

I'd like to know how proper proper torque value is determined, not start any arguments or otherwise. Sorry that I have to ask why, but I never stopped doing that. I'm waiting for my 2 year old daughter to start.

Greg

You hit upon a good poimt. Most of the small engine manufacturers DO NOT required clamping forces to provide a bolt torque appropriate for the task. If people compared the size screws used on the hubs to torque tables they would find that only the torque value for the particular grade of screw was used for clamping force torque.

Worse, little or nothing is factored into the torque value for prop composition or for the base material receiving the screw. A steel screw being torqued into an aluminum hub can run into some serious thread deformation issues on the aluminum side when a steel screw approaches peak torque values. Short engagement screws would make that happen in a hurry.

Remember that full scale props also have safety wires limiting bolts and nuts from backing off. Our little engines do not receive the same treatment so loading the screw is the method used for fastener security. Not the best way but it is functional. I don't want to think about the cries for mercy if all the new and old gas engine users suddenly had to start using safety wire.... I can hear it now.."Who's got the best price on safety wire pliers", or "can I only do 4 twists per inch?", or "does it really need to be .032" wire?", or "do you have to twist right hand or left hand?"......lol.

Um, I have the safety wire pliers and the safety wire, but where do I find the cross drilled cap screws used with the safety wire in the sizes we use?

Thanks

Richard

McMaster

Pat - couldn't see metric ones listed of any size.....c'mon now, don't hold out on us???? Where can I get these so I can be the first guy at the field to safety wire my 100cc ??????????
I haven't had to do any good safety-wiring in quite awhile.....

Somebody says its a" friction" thing for prop to not slip....why do many of the multi bolt hubs use smooth as a surface. Whats up with that?A good knurl...not to deep...seems like thatwould help????? Capt,n

A knurl adds bite to the friction. A leather pad increases the friction. Both result in less needed clamping force.

Greg, The 50cc has M5 bolts, but only three of them are used / supplied with the engine. The 58 has 6x M4 bolts. These bolts are not torqued to the bolt torque limit. My starting point would be 60% of that at most.
On all engines, a slight amount of wood crushing should be the torque limit. If the prop then slips, the wood is too soft and should not have been used for propellers. Consequently, it should be removed from your engine, because sooner or later the prop bolts will fail.