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Old 11th Dec 2011, 5:24 pm   #1
Skywave
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Arrow Tuning gearbox for an AR88D

As per title; an AR88LF 'box may also be suitable, though. The one I have is clearly beyond repair.

Thanks.

Al. / Dec. 11th., 2011
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Old 20th Dec 2011, 10:29 am   #2
Mike Phelan
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Default Re: Tuning gearbox for an AR88D

What's actually wrong with it, Al? Pics?
AFAIK the LF 'box will be the same as the D.
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Old 20th Dec 2011, 3:48 pm   #3
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Arrow Re: Tuning gearbox for an AR88D

Hi Mike: thanks for your response.

Basically, excessive backlash*. I shall précis the whole story up to now by stating that the usual 'strip, clean, adjust, lubricate and re-assemble' procedure has been carried out: there was some improvement. The bearing on the input shaft is also quite worn and 'wobbly'. I assume that the cause of these troubles is primarily simply excessive overall wear in all the mechanics.

The 'box is now re-fitted, so photos. of it are not really possible. However, the current Thread by Gezza, with its photos., applies.

* By my use of the word 'backlash', I refer to the phenomenon whereby when the main tuning knob is rotated, there is a dead space before the tuning capacitor starts to move. When the tuning knob is rotated in the other direction, the 'dead space' effect repeats. Personally, I think a better word to describe this would be 'hysteresis', but if 'backlash' is the generally accepted name for this effect . . . so be it.



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Old 20th Dec 2011, 5:48 pm   #4
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Default Re: Tuning gearbox for an AR88D

Aren't all the gears double and sprung to take up the backlash, Al?

I must admit it's been a long while since I owned both types of AR88 so might be wrong on that.

Update - seen Gezza's pics here and they are like that; it's necessary to compress the springs before meshing each pair of gears with its pinion on the previous stage.

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Old 20th Dec 2011, 7:53 pm   #5
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Default Re: Tuning gearbox for an AR88D

Yes, Mike, the large gears are anti-backlash types. I was deliberately brief in my reply to your first questioning post; I'll now expand.

I made sure that these gears weren't sticking on account of dried up grease; fitted two springs that were missing; checked that each pair of gears would 'move back', as they were moved against each other, opposing their springs' force; checked their teeth for any signs of visible excessive wear: hooked teeth, etc. Then re-assembled the entire unit.

I did discover that at the point where these gears engage to the smaller pinions, there was excessive wear of the teeth on those pinions. However, by adjusting the end-float of the anti-backlash gears, I was able to introduce sufficient lateral movement so that the teeth of the anti-backlash gears engaged with unworn teeth of their respective pinions.

So, to summarise: I've done everything I can think of to solve the problem - which remains. I can only surmise that the cause is simply down to net accumulated wear in the 'box from input shaft to the output shaft that is coupled to the tuning condenser.

Taking the 'box apart and its consequent re-assembly is a difficult task; I doubt that repeating the above procedure will bring about a substantial improvement. Hence, my search for a replacement box.

Al.
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Old 20th Dec 2011, 9:12 pm   #6
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Default Re: Tuning gearbox for an AR88D

Quote:
Originally Posted by Skywave View Post
... by adjusting the end-float of the anti-backlash gears, I was able to introduce sufficient lateral movement so that the teeth of the anti-backlash gears engaged with unworn teeth of their respective pinions.
Hi Al, this is exactly what I did with mine.
Mind you don't pre-load the anti backlash springs too much, or it will quickly wear out again.
I lubricated the gears with moly grease to try to reduce wear. It really does need an EP lubricant since the contact area of the gear mesh is very small.
Being steel, they act more like circular saw blades.
Rob.
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Old 20th Dec 2011, 10:01 pm   #7
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Question Re: Tuning gearbox for an AR88D

O.K., Rob - thanks.

Since I found that two springs were missing, I fitted two springs from my 'assorted springs box' with what I could find and would fit the gears. And that has started me wondering. Could the problem largely be due to the original springs that are in there simply being now tired and in need of replacement? And if so, where can I find suitable ones, 'cause they're very small?

Al.
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Old 20th Dec 2011, 10:32 pm   #8
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Default Re: Tuning gearbox for an AR88D

Hello Al, it should be relatively easy to check the springs, you probably know this but may be some are unaware.
When rotating the tuning knob back and forth the offset difference between the pair of split gears shoud not hardly vary, if it does the causes can usually be traced to excesive end play on shafts or stiffness within the gearbox train, also a stiff tuning gang can give the same symptoms, reduced clutch spring tension can also give a backlash effect.
I have not delved into mine yet but on other receivers I have done where the springs are in a slot in the gears ie: under compression, the split gear springs are almost under full compression when correctly seated.

Hope this helps someone.

Lawrence.
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Old 21st Dec 2011, 12:30 am   #9
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Default Re: Tuning gearbox for an AR88D

Quote:
Originally Posted by Skywave View Post
When the tuning knob is rotated in the other direction, the 'dead space' effect repeats. Personally, I think a better word to describe this would be 'hysteresis', but if 'backlash' is the generally accepted name for this effect . . . so be it.
I think of hysteresis as referring to a system that jumps rapidly between two different saturated output states as a result of positive feedback, with the trigger point moving in the opposite direction to the stimulus. Backlash is just a dead band when the stimulus reverses, there is no sudden change in the output as the stimulus continues, just a linear response.
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Old 21st Dec 2011, 12:54 am   #10
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Default Re: Tuning gearbox for an AR88D

That post is drifting this Thread OT, but I'll respond to it anyway.

In the B/H curve for a magnetic material, the resultant value of the flux density, B, is dependent on the magnitude and direction of the magnetising force, H ~ the phenomenon commonly known as 'hysteresis'. (A state of saturation is not a pre-requisite for this effect.) It is quite evident that this effect is, in practice, very similar to what is referred to as 'backlash' in gear train mechanisms: the value of the output variable is dependent on the direction and magnitude of the input variable.

In rotating mechanisms, be they employing gears or pulleys, it is possible for a problem to develop whereby as a shaft is rotated in one direction, a force develops that tries to rotate that shaft in the opposite direction. To me, a sensible description of that force is 'backlash', since it literally 'lashes back'. There is no 'dead space' with this effect.

Al.
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Old 21st Dec 2011, 1:14 am   #11
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Default Re: Tuning gearbox for an AR88D

A little out of focus I'm afraid, but gives the idea.
Rob.
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Old 21st Dec 2011, 10:28 am   #12
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Default Re: Tuning gearbox for an AR88D

To keep on topic surely the purpose of the split gears and compressed springs is to ensure there is some meshing of the gears. When the control knob is rotated there is not a "dead band" whereby the output of the gearbox ( driving the dial cursor ) does not move until the slack in the gears is taken up. Idealy there has to be some clearance in the gears and this is accomodated by the compressed springs ( auto variable meshing if you will ).

Clearly bad backlash is very obvious to the user when trying to resolve a SSB signal as you have to tune past the signal far enough so that when you reverse to come back the gearing is in contant mesh ( dare I say a linear output from the gearbox )

Personally hysteresis and backlash surely are the same because when the effect is occuring there is a non linear response from the output of the gearbox to that of the input.

Mike
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Old 21st Dec 2011, 10:33 am   #13
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Default Re: Tuning gearbox for an AR88D

Al
Is the wear purely the teeth or also the pivots and their holes?

Putting my horologist's hat on, a few comments:
  • Steel against steel in exposed gearing is not good, that's why clock wheels are brass and pinions are steel.
  • The steel always wears before the brass as any abrasive dust embeds itself in the softer material.
  • Exposed clock gearing is never lubricated as it picks up dust and forms an abrasive paste. Were the AR88 tuning drives originally lubricated?
  • Worn pivots and holes can be repaired by bushing.
If it were me, I'd try for a replacement drive and keep the old one, repairing it.
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Old 21st Dec 2011, 10:35 am   #14
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Default Re: Tuning gearbox for an AR88D

Al.

Have you tried locking the output shaft by some means, then observing the mechanism whilst gently rotating the input shaft? This should enable you to determine where the play is.

Does the box use plain or ball bearings?
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Old 21st Dec 2011, 12:07 pm   #15
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Default Re: Tuning gearbox for an AR88D

Trouble is with these sets is they are a knob spinners delight, the initial inertia required to get 'em spinning puts a lot of strain / wear on the input shaft and its associated bearings, gears etc. Unless it is something simple then from what Al has descibed so far it seems to point to accumalative wear throughout the box ie: it's shot.
I have the same problem on another receiver, a good second hand or new old stock is my only hope.

Lawrence.
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Old 21st Dec 2011, 12:49 pm   #16
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Arrow Re: Tuning gearbox for an AR88D

Graham:

Answer to your first Q.: no.
Answer to your second Q.: from memory . . .
input shaft has ball bearings. Other shafts have plain bearings and a single ball at each end for end-float adjustment.

Al.
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Old 21st Dec 2011, 12:56 pm   #17
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Default Re: Tuning gearbox for an AR88D

Mike,
The comments you make about such mechanisms are things that I would never have considered: to me, those comments are of value. But then my knowledge and experience of clocks and similar things - unlike yours - is just about zero!

Al.
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Old 21st Dec 2011, 1:44 pm   #18
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Arrow Re: Tuning gearbox for an AR88D

Mike: you have a P.M.

Al. / Dec. 21st. //
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Old 24th Dec 2011, 12:17 am   #19
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Question Re: Tuning gearbox for an AR88D

Having had a further think and a play with this receiver, I've come to the conclusion that a likely cause of the backlash is simply insufficient compression of the springs in the anti-backlash gears. So, to that end, I have removed the box (again) and have it dismantled (again) on the bench. Now I have a problem: what is the trick of adjusting the springs compression (which is difficult enough) and then refitting the gears back into the box so that the tensioned gears simply don't fly back to their original uncompressed state? Obviously one of the side-plates has to be removed to extract the gears for adjustment and subsequent refit. Therefore a gear, when fitted and adjusted, is free to jump out of mesh with its accompanying pinion - and the springs thus simply expand! Is there some special tool required for this?
What's the trick, guys, 'cause it sure beats me!

Al.
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Old 24th Dec 2011, 10:09 am   #20
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Default Re: Tuning gearbox for an AR88D

Not easy, Al. Apart from making a suitable tool, there might be room to get a few toolmakers' clamps in.
Like these.
Another possibility if you don't mind small holes in the gears is to tension each one before you put it in the frame, clamp it in a vice and drill a small (2mm maybe) hole through the pair and put a small pin in. Once assembled, pull all the pins out.
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