Nord-Lock Washers with Wooden Prop

[ahramin]

Can Nord-Lock washers be used for prop bolts on a wooden prop or must the bolts be lockwired?

[Marc Zeitlin]

Can Nord-Lock washers be used for prop bolts on a wooden prop or must the bolts be lockwired?Since wood prop hubs can shrink when they dry out, a washer that depends on compression for anti-rotation would not be useful. Now, as it turns out, safety wire on wood props isn't all that useful either, since it does nothing to maintain compression but only prevents the bolts from turning.

If you're using a wood prop, what you want (if you have a diameter bolt for which I've engineered a solution) is belleville washers. See:

http://cozybuilders.org/Prop_Bolt_Bellville_Washer/index.html

for more information. Feel free to contact me directly if you have any questions. But even with the bellevilles, safety wire is required. It might be interesting to try Nord-Locks in combination with bellevilles instead of safety wire - I'll have to think about that a bit more, but it should work as a safety wire replacement in that instance.

[ahramin]

Very interesting Marc. Thank you.

[melann]

Since wood prop hubs can shrink when they dry out, a washer that depends on compression for anti-rotation would not be useful. Now, as it turns out, safety wire on wood props isn't all that useful either, since it does nothing to maintain compression but only prevents the bolts from turning.

While I agree that safety wire will not maintain compression on the bolts, it WILL prevent them from departing the aircraft.

[Marc Zeitlin]

While I agree that safety wire will not maintain compression on the bolts, it WILL prevent them from departing the aircraft.Nope - didn't on my plane. See:

http://cozybuilders.org/Oshkosh_Presentations/2007_Prop_Loss_Desert_Center.pdf

The safety wire will prevent the bolts from turning, but that's not worth diddly squat if the hub shrinks away from the crush plate and the hub loses compression. Prop starts moving back and forth on the drive flange, the bolts fatigue, and bam - gone.

[Bill Berson]

Interesting report. I guess it glides well with no prop.....

What is "static torque"? (from report: "only checked static torque")

edit, I found from your other webpage that said loosen the prop bolts first, then retorque while rotating in motion. Is that procedure adequate in most cases?

[Marc Zeitlin]

Interesting report. I guess it glides well with no prop.....Better than with it windmilling, yeah. And smooth, too :-).


What is "static torque"? (from report: "only checked static torque")

edit, I found from your other webpage that said loosen the prop bolts first, then retorque while rotating in motion. Is that procedure adequate in most cases?Yes. There are always inaccuracies in torque measurements, due to friction, surface finish, lubrication, interference fits, etc., but if the bolt isn't rotating while you're measuring the torque, you're pretty much guaranteed to get a very inaccurate number unless you've lubricated the hell out of the threads first. Which you can't do if the prop's been on the plane for 6 months to a year prior to the torque measurement :-).

[ahramin]

Every oil change (4 months) I remove the bolts, clean and lubricate with beeswax, and reinstall. This is the only way I know of getting an somewhat accurate preload on prop bolts.

It would be nice to use lock washers instead of lockwire to speed up the process.

[Marc Zeitlin]

Every oil change (4 months) I remove the bolts, clean and lubricate with beeswax, and reinstall. This is the only way I know of getting an somewhat accurate preload on prop bolts.And if you used Belleville washers, you wouldn't need to do this but once per year, at the Condition Inspection.


It would be nice to use lock washers instead of lockwire to speed up the process.You indicated that you read my response above - while it would be nice, without bellevilles the Nord-Locks would be WORSE than safety wire, as a loss of compression could also lead to bolt rotation - at least the safety wire doesn't allow that, for what it's worth (which is little).

[Bill Greenwood]

Is this a case of the prop manufacter using "green" instead of seaoned wood? I have use two wooden props and never had any slack or slop when rechecking the torque on them. My Sensinich on the Cub is nicely varnihed and seems to be waterproof. I cant recall the spec ,but it is fairly low maybe 25lbs so you are not crushing the wood. I also have a metal hub with 4 wooden blades with metal sleeves threaded into the hub and doesn't seem to have problems. Dowty Rotol design.

[flyrgreen]

I know that this thread is about wooden props/Nordlock washers, but for interest-- Sensinich sends Nordlocks with their composite props when you buy a new one. They not only claim a zero fail rate, they put it in writing. In 4 years mine have never lost torque spec. I don't know what their deal is for wood props.

[Dana]

A new wood prop installation is supposed to be retorqued after the first hour, then after 25 hours, and every 50 hours thereafter. Torque on 3/8" bolts is 15-19 ft-lb. From the Sensenich installation instructions:


The main factor that leads to the loss of propeller bolt torque is the variation of the wood hub thickness. The hub thickness will vary with (a) wood moisture content changes and (b) temperature changes. Even though your propeller has been sealed and painted, changes in wood moisture content will occur which can significantly change the thickness of the hub. A one percent (1%) change in the moisture content of a propeller (increase / decrease) will cause a 0.010” change in hub thickness. As the required compression for a typical 65 HP wood propeller is 0.021”, almost half of the required hub compression has now been lost. Moisture content changes are not immediate and can span several weeks or months, depending on many factors such as temperature, humidity, and operating schedules.
Operating temperature changes have similar effects but are not as severe.

[Bill Berson]

My variable pitch Hoffmann prop has fiber nuts. (1/2", I think.)
I saw a charred wood prop that was loose.

[Marc Zeitlin]

A new wood prop installation is supposed to be retorqued after the first hour, then after 25 hours, and every 50 hours thereafter...When I was researching for the belleville washer design, I found requirements from many wood prop MFG's that ranged (after the first retorquing or two) anywhere from every 10 hours (holy crap - that's all you'd spend your time doing) to the 50 hours you state. The most interesting one was Jabiru - they originally had a 25 hour retorque time, but after they went to belleville stacks on their prop bolts for their wooden props, they dropped the maintenance interval to once/year, no matter how many hours the plane flew.

That's what I use for aircraft that use bellevilles - inspect/retorque at the CI only (or whenever the prop is removed and reinstalled, obviously).

[melann]

One thing I haven't seen mentioned is seasonal issues. If you live in an area where seasons change from extremely wet to extremely dry, then you should retorque a wood prop when these changes occur. I don't mean to imply every time it rains, but at the beginning and end of such seasons.

[Airmutt]

Not just seasonal but a big swing in temperature; obviously warm to cold where the wood can contract. Note to self......Probably not a bad idea to record the temp and RH with each retorque. Just thinking out loud.

[melann]

Not just seasonal but a big swing in temperature; obviously warm to cold where the wood can contract. Note to self......Probably not a bad idea to record the temp and RH with each retorque. Just thinking out loud.
Not a bad idea.