Prop bolt fragility

explanations on the use of Article M7DT

We have faced this problem sometimes

The fragility of the screws always occurs at the point indicated in the drawing,

the base of the nut, both with wooden and carbon fiber propellers.

That problem may happen with heavy or light wood props, carbon props, with or without rubber or spacers, withour an apparent logic relation.

What surprize us is that the same flange design was copied from other manufacturer, with more powerfull engines with more vibrations and they don't have this kind of problem.

The fact that some clients, happens much more frequently makes us believe that the system to tighten the screws is very important, in most cases using the torque wrench the problem is resolved.

We are sure most of the times the screws are not tightened at the correct tension.

Sometime we verify that the torque wrench was not used properly by the customers, please ask to your hardware supplyer or reed the manual before to use it.

We make many test in our shop and we NEVER verify that problem.

Unbalanced props, or tightening without follow our “verify the tracking” suggestion in our manual can multiply the risks to have this problem.

On the last prop hub since year 2010 we have 4 holes of 6 mm and 4 holes of 8 mm for the prop bolts, please verify you use the right one !

 The problem is not common, but there were several customers who have reported, so we propose the following recommendations and solutions to customers that really are experiencing these problems. We insist that there is no need to worry, the ruptures are rare, we believe it is sufficient to properly follow the procedures outlined in the user manual.

SOLUTIONS

If you are really sure they are correctly tightened and you still have that problem there are some solutions :

• use different hardness grade of bolts, + or - hard, stainless (softer)or 10-9 grade (harder).
  As we make many tests and believe we use the right grade of bolts We don't suggest to replace ALL the bolts with different grade.
  In this case we suggest to change only 2 bolts a time and cross it, 2 standard bolts in the opposite holes + 2 in cross.
  Stainless bolts or harder bolts have different fatigue behavior.

• Use 8 mm bolts, we really don't believe is necessary, but if you really scare about that fragility, this can be a solution (possible only on the prop hub and flange delivered since year 2010).

• verify if you put the 6mm bolts in the right hole size ...

• Than the solution we prefere, but a little bit more difficult to do, because you must modify the propeller holes in each of the prop you will use :

with the nuts M7DT : visit the shop   you find it in the reductor section of our shop This solution is copied from aircraft design.

Due to the design of the nut the stress is distributed over a larger area and eliminates the component of bending load that should be the cause of breakage of the screws.This solution allows the use of shorter screws, in this way it compensates for the weight gain and the result is very pleasant, not having screws which protrude from the hub.To assemble these nuts are use the holes of the hub 8 mm that are standard in our production already done since 2010.You must modify each propeller, enlarging the holes from 6.2 mm to 8 mm because the nut must penetrate about 2 cm in the propeller hub.To do this we have prepared a very simple tool to use that selfcenter on existing holes, you better be very careful, however, to the perpendicular of the hole, so we recommend using a drill press.

Article MANFE : milling tool in our onlineshop maintenance/tool section

 is important then to use the correct size of bolts, which must not protrude excessively and should work without getting to the end of the threaded part.