converting mechanical retracts to electric screw operation

[Bob Merrett]

While working on my 1/12 scale Gloster F9.37, I have been looking at ways of converting mechanical retracts from direct servo to electric screw operation. “Why?” you may well ask. Well, I had mechanicals ready to use, didn’t want the snap they give, didn’t want to spend even more money on the new servoless ones that have come on the market and betted myself that I could get some everyday available items and make it happen, cheaper. I know things have moved on now and they can be bought for a very reasonable price, but I thought I’d still share some ideas as it may help you dabble or adapt for some other application.

Option 1. All in one miniature geared motor, coupled to a threaded rod (please see cheap shaft coupler) and trunnion and microswitch operation.

The motor cost £5, and as I have a tap and die set, I extended the thread that is already there on a 3mm coupling rod (that I already had, as will you too no doubt). Make up your own trunnion from something like a nylon adjustable aileron connector, 3mm t-nut, 3mm hex nut, 3mm thread insert in to wood or nylon, anything really that will wind its way up the thread. 3mm thread diameter is plenty tough enough for this operation and is easy to calculate the linear travel required.

Example. The existing actuating rod and trunnion travel 25mm from up end to down end. The thread pitch is a very handy 0.5mm or 0.5mm linear movement for every revolution, two revolutions to make 1mm of linear movement. Still with me? Good. I was after something like a four or five second cycle time from down to up (and up to down of course), but how was I going to achieve this?

Well, just substitute your values in to this formula, and away you go.

Travel Length (say 25 mm) ÷ Time (in 5 seconds) ÷ Thread Pitch (M3=0.5) x 60 = RPM speed required from motor.

25 ÷ 5 = 5 ÷ 0.5 X 60 = 600RPM operation speed to achieve this.

Let’s say the ungeared motor turns at 6000RPM, then I need a reduction gear ratio of 10:1 (12000RPM being 20:1 ratio). Motor RPM ÷ operational RPM. Easy mathematics I know, but you get drift.

3mm thread is also great for using as a fine tuning and adjusting thread as a half-turn would be 0.25mm of movement and a quarter-turn would be 0.125mm of movement etc. etc. etc.

Option 2. Miniature motor (pack of 5 for £4), worm drive reduction with an internal thread, threaded rod coupled to existing actuating arm (via a fixed clevis in this case) and microswitch operation. Worm drives are brilliant and offer the highest single cog ratio reduction, plus they naturally form their own brake when not in motion.

For that slightly bigger project just move things up a notch to M4. I found some M4 centre hole furniture cross dowels that would make perfect metal actuating trunnions, and they come in different lengths, just check out e-bay.

[GrahamC]

That's very clever, and way beyond what I can do....

I guess I'm glad that the hobbyking style ones work really well (and keep working) A pair less than £15 delivered, and I saw someone selling the same units at the NATS, and they had bigger units too - up to 8KG if I remember rightly.

I'm using the 4.5K rated ones in a 62 inch Spitfire (and building them into a similar sized Zero) and the smaller 2.5K ones retracting backwards in a 46 inch Mosquito. Amazing value and reliability.

[Bob Merrett]

Thank you for your kind words Graham. If i'm going to be honest, then I wanted to prove to myself that it could be done, and in fact, that I could do it. I am no engineer and have no special tools, just your average builder really with general tools, who just wanted to try something a little different. I just thought I'd post the idea as somebody might be able to use/adapt it for a undercarriage, swing wings etc on a project they might be doing or thinking of doing. It just means that you can dictacte how fast or slow you want something to operate.

Another confession Graham. I too have now bought (I'm guessing) the same pair as you as the the prices have really dropped since i firsted the project. All right, all right, stop verbally abusing me. I keep changing loads of things as I go, and this was one of them. In the end I opted for commercially available stuff (the price helped) instead of bespoke, just in case others liked the model and wanted to build it.