Ballscrew Balls-Up


After about 200hrs I measured the original screwjack backlash as per Sequoia recommendation and noted it had got to about half the allowed tolerance. I could see myself getting about another 100hrs from them and then they would need replacing.

I considered it was time to update the system slightly. I didn’t want  to change the whole thing but having spent 20 years walking around airliners which use ballscrews to actuate flaps and things I thought the change to ball screws would be a simple enough upgrade for a significant improvement.

I went back to the original supplier of my screwjack parts for the new components. The parts I selected were new 14mm diameter ballscrews with the appropriate ball nuts. The lead on the ballscrews is 4mm which is close to the original system. While Sequoia’s gear motor was designed for the smaller lead the torque required to actuate the ballscrews is significantly less than the Acme threaded screwjacks so I considered it a good move. Two of the benefits would be that the wear on the system would be just over half and the gear cycle would be just over half the time too.. Great ! 

Sourcing the parts seemed to be a lot more difficult that in should be so I decided to revisit my selection from other manufacturers considering size options, local availability and price.

What I decided on was 12 mm diameter  ball screws for the nose gear and 16 mm ball screws for the mains. The larger screws are a more standard size and considering the original production screws are close to this then I think the extra size shouldn’t be a problem. The nose gear is a problem in that there is very little room in the well so I stuck with 12 mm. 

I also selected to go back to the production Falco lead of 5 mm. This will get the gear up in half the time. Of course the torque required to raise the gear is more with the coarser thread but when the Sequoia Falcos had 10 tpi specified the efficiency dropped off to about 34% so with the 95% efficiency of the ball screws there should be no problem with the 5 mm lead.

17th September 2008 … the new ball screws arrived.

The 16 mm screws are considerably heavier but I think they are worth the hassle.. we’ll see.

Another consideration is that the ball screws will back drive. That is, with longitudinal force applied to the  nut, the screw will turn.. this is of course everything working in reverse and not desirable but a side effect of the ball screws being so efficient. The upshot of all this is that there must be some sort of lock built into the system to hold the gear up and down when there is no power on the gear motor. The lock must consider the emergency case also so the gear can be lowered and locked down if the system fails. It must re-engage when the gear is down to stop the jack springs from winding off the locking pressure.

The system I am considering will be attached to the motor release knob so the motor and lock get disengaged together. We’ll see how I get on with this in time.

I drew up a CAD drawing of the nut adapters I needed and sent them off to my engineer with a lump of 4130 bar. 

22nd October.

I have just got the 16mm nut adapters back from the engineer. Welding is next. The holes around the barrel of the adapter is for rosette welds.

 The threaded section will also be drilled and tapped for 2 or 3 set screws to stop them unwinding at an inopportune moment.

I admit it.. I’ve been slack. I haven’t had a huge amount of enthusiasm for this of late but when Daylight saving finished I realised that I better hurry up if I want to install them this winter so I did a little work on the jacks.

 Here are the jack sleeves after initial assembly.. I still need to drill holes to size, tap the gear door arm holes and drill and tap the set screw holes .. oh and paint them after a general tidy up of course but here they are. The large versions for the main gear seem huge.. hopefully not too huge !  16 April 2009

This is the end of the small screw before machining down.. this is the way I did the original ACME screws and it seemed to work so I thought I’d try it again. Just fill the hollows in the thread with welded filler material. 16 April 2009 

Here are the screws ready for install. The adapters have set screws in to help hold everything together along with thread lock.

The screws have the ends complete. Just a couple of holes to be drilled and reamed in the ends.  12-May-2009

Now I need to make the gear lock.. I have got it basically set in my mind.. just need to start making bits. 

In the end, I gave up on this. To do it properly it really needs a complete re-design.


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