Prop bolt shear on M600

[Bobby A.]

Guys
Have had the top of the prop bolts shear off on a M600 work bird. Bolt stripes still aligned, no deviation, no over tightening, no modifications. Has occurred twice on same bird twice in two weeks. Has anyone else had this issue? And of course like everyone else with an in-house maintenance program we have questions about the factory torque specs groom DJI. Anyone have that info?
Thanks

 

[mick]

Guys
Have had the top of the prop bolts shear off on a M600 work bird. Bolt stripes still aligned, no deviation, no over tightening, no modifications. Has occurred twice on same bird twice in two weeks. Has anyone else had this issue? And of course like everyone else with an in-house maintenance program we have questions about the factory torque specs groom DJI. Anyone have that info?
Thanks

Are the motors all running smoothly pal? We always put a stick or screwdriver in our ear and touch the motor housing to hear the vibration.It works brilliant. I have done this trick for years checking plant systems and motors etc. You might find a motor is causing excess vibration as its on its way out. Our inspire motors seem fine but i can detect a growl in some with the ear test.

 

[mick]

Are the motors all running smoothly pal? We always put a stick or screwdriver in our ear and touch the motor housing to hear the vibration.It works brilliant. I have done this trick for years checking plant systems and motors etc. You might find a motor is causing excess vibration as its on its way out. Our inspire motors seem fine but i can detect a growl in some with the ear test.

All that said it would not surprise me if Dji are to blame some how, Dji use appalling quality bolts imho

 

[Bobby A.]

Are the motors all running smoothly pal? We always put a stick or screwdriver in our ear and touch the motor housing to hear the vibration.It works brilliant. I have done this trick for years checking plant systems and motors etc. You might find a motor is causing excess vibration as its on its way out. Our inspire motors seem fine but i can detect a growl in some with the ear test.

Thanks for the response Mick, will give the stick/ screwdriver trick a shot in the morning. I've been keeping close tabs on all our motors via touch and feel but Will give the hearing test a try. We also after 10 to 12 flights let the bird sit at idle for a minute after landing and shoot each motor with a laser thermal gun. I had slightly higher temp readings on motors as the bearings start to go. This bird has over 300 flights so right now I'm leaning toward end of life expectancy of certain components due to fatigue so I'm going to do a major replacement of many components and monitor it very closely.

 

[Moosewax]

I have had the same issue occur on an M600 Pro, twice.

Also recently been noticing very small hairline cracks underneath the trailing edge of the propellers. They did not make contact with anything so I have recently been trying to figure out if its just age, or stress when transporting etc.

The cracks are VERY easy to miss, have to either inspect with a flashlight while bending the prop slightly, or with fingernails to catch some of these. They only seem to occur on the bottom of the props ("bottom" when mounted). Have had this occur with multiple sets of props.

Heavy or light payload seems to not make a difference.

 

[PatR]

If you are not using a torque driver to tighten prop screws you are very likely over torquing them and setting them with unequal torque. Both will cause them to break. Over torquing places high stress loads on the screws while unequal torque causes the prop to wobble at the hub, which will shear the screw shaft.

As manufacturers do not provide torque specs we have to review engineering screw torque tables for the size and type screws being used for dafe torque values. As I don’t know what size or material your screws are I won’t post a torque value but it would be surprising to see a value greater than 6 inch pounds.

Also, Chinese manufacturers use deplorable quality mechanical fasteners. A wise person will peruse good quality hardware suppliers and obtain better quality fasteners for flight critical locations. Stainless steel screws are usually the best.

 

[Moosewax]

If you are not using a torque driver to tighten prop screws you are very likely over torquing them and setting them with unequal torque. Both will cause them to break. Over torquing places high stress loads on the screws while unequal torque causes the prop to wobble at the hub, which will shear the screw shaft.

As manufacturers do not provide torque specs we have to review engineering screw torque tables for the size and type screws being used for dafe torque values. As I don’t know what size or material your screws are I won’t post a torque value but it would be surprising to see a value greater than 6 inch pounds.

Also, Chinese manufacturers use deplorable quality mechanical fasteners. A wise person will peruse good quality hardware suppliers and obtain better quality fasteners for flight critical locations. Stainless steel screws are usually the best.

This post is almost a year old, in that time thankfully we have not seen this issue reoccur across 3 different M600s.

According to my local drone dealer other people have seen this issue occur in the wild.

We don't have torque values for these screws but we tighten the screws to where the props can rotate in their bracket with some resistance, rather than rotating freely or easily within their brackets. Basically if I rotate the props by hand and stop them manually quickly, if the props flop out of place then they are too loose.

I believe this may help to prevent wobbling as described by PatR from too low of a torque.

 

[PatR]

Realizing this is an old thread but recognizing this is a continuing issue I’ll simply say that any bolt tightening method that does not employ a quantifiable torque value cannot be consistently repeatable and provides zero assurance of functioning correctly for every flight.

Aviation is not an activity where an attitude of “I think, or hope, that will work” has any validity. There’s no reason to be a test pilot for every flight.

 

[Moosewax]

Realizing this is an old thread but recognizing this is a continuing issue I’ll simply say that any bolt tightening method that does not employ a quantifiable torque value cannot be consistently repeatable and provides zero assurance of functioning correctly for every flight.

Aviation is not an activity where an attitude of “I think, or hope, that will work” has any validity. There’s no reason to be a test pilot for every flight.

What kind of hex tool can be used for low torque settings?

Everything I can find so far is 10lbs+ and most is 20lbs+ on their lowest setting.

 

[PatR]

Go online and research “torque driver”. You’ll find quite a few that measure in very low force ranges. Drivers that can measure as low as only a couple inch pounds are common, and even those that can measure in inch ounces can be obtained. They are not cheap but are invaluable for small fasteners. One brand name that quickly comes to mind is WiHa. There are many others.

 

[Moosewax]

Go online and research “torque driver”. You’ll find quite a few that measure in very low force ranges. Drivers that can measure as low as only a couple inch pounds are common, and even those that can measure in inch ounces can be obtained. They are not cheap but are invaluable for small fasteners. One brand name that quickly comes to mind is WiHa. There are many others.

Yeah torque driver was the key word apparently. I was looking up torque wrenches originally.

Any recommendation on a range?

Found one that does 3 - 15lbs, or 5-40 lbs.

I would imagine the 3-15lbs would be best for multirotors / small electronics.

 

[PatR]

Steer clear of anything that provides foot pounds unless you're working on your car. Most everything we "hobbyists" make use of utilizes ranges measured in inch pounds, usually under 20in.lb. m3 prop bolts are in the area of 5 or 6in.lb. I'd have to visit the look up tables to be certain of that.

Note what type of torque values a tool provides. Newton meters, foot pounds, inch pounds, and inch ounces are the most common. Buy the tool that matches the type of torque value used by the fastener size and material.

 

[Moosewax]

Steer clear of anything that provides foot pounds unless you're working on your car. Most everything we "hobbyists" make use of utilizes ranges measured in inch pounds, usually under 20in.lb. m3 prop bolts are in the area of 5 or 6in.lb. I'd have to visit the look up tables to be certain of that.

Note what type of torque values a tool provides. Newton meters, foot pounds, inch pounds, and inch ounces are the most common. Buy the tool that matches the type of torque value used by the fastener size and material.

Thanks for the responses PatR I will look into this further. If I can get a definite answer on a manufacturer recommended torque setting for the parts in question on this post I will post it here.

 

[drummingchevy]

Are the motors all running smoothly pal? We always put a stick or screwdriver in our ear and touch the motor housing to hear the vibration.It works brilliant. I have done this trick for years checking plant systems and motors etc. You might find a motor is causing excess vibration as its on its way out. Our inspire motors seem fine but i can detect a growl in some with the ear test.

This poster hasn't been active in some time but... does anyone know what this looks like? I am trying to picture it but I can't. I'm becoming increasingly proactive with maintenance on my drones and all I do is look to see that the motors spin smoothly.