How "slipping" clutch works to regulate takeup reel speed in cassette deck

[MarkoGVA]

Hi all,
I am looking to understand the mechanics of how the "slipping" clutch works in regulating the speed of a takeup reel in cassette tape players.
Does anybody have some technical notes or explanations?
How to calculate the spring force required to slow takeup reel speed as the reel fills up with tape?
Thanks!

[Ian.M]

The tape speed is controlled by the pinch roller against the capstan, not by the slipping clutch.  The slipping clutch only has to limit the torque so it doesn't exceed the tape's tensile strength with an empty takeup reel, and be driven fast enough to keep up with the capstan when the reel is empty.  As the reel fills, the tension decreases due to the increasing diameter and the clutch slips more as the reel rotational speed decreases.

[MarkoGVA]

Thanks!
But say the clutch is not working correctly....how to calculate if the spring is applying the "right" force?

[Ian.M]

Got a tape tension gauge?  If not, at best you are guessing.  If it doesn't stretch the tape with an empty reel or fail to take up when the reel's nearly full, you are 'in the ballpark'.

[MarkoGVA]

True...
I wonder if there are any schematics or a diagram of the working clutch?
Like something out of an old "patent" application.

[MathWizard]

Over the years, how similar were tape players to each other,  and likewise with VCR mechanisms, did most companies make them about the same ? Or was it a variety show of mechanisms ?

[floobydust]

The clutch does not regulate speed. That is the job of the motor at the capstan. The takeup wheel is always overdriven speed-wise and so the clutch is always slipping to maintain tension.
It is typically a cloth disk compressed between two plastic disks, like a miniature car clutch. It depends on the quality of the tape mechanism, cheap or high-end, auto-reverse etc.

Back in the day used a takeup torque gauge but eventually you can put your finger on the hub during playback and judge if there is enough takeup torque. Required highest on cheap and long i.e. C-120 cassettes.

If the deck falsely auto-stops, you can see the takeup hub stop turning now and then. It can also be caused by belt slippage, or an idler wheel slipping- whatever drives the takeup wheel. Or the hub sensors (magnet+reed switch) not working or cutting out. It will "eat tapes" if the takeup hub is too slow, makes a mess.

To fix them, remove the plastic C-clip and take out the hub with clutch assembly. Don't lose any washers.
We would dunk and soak them a while in IPA, turn the clutch, and pull the wheels apart and sometimes clean in there. But rarely, a clutch will wear out, or get oil/grease contamination on the cloth disk. Then dry out the clutch and the torque is usually much better.
You have to be careful here, any oil/grease will easily get in the clutch or the belt or idler pulley. It only goes on the metal pin if you lube it during reassembly.

[shakalnokturn]

Over the years, how similar were tape players to each other,  and likewise with VCR mechanisms, did most companies make them about the same ? Or was it a variety show of mechanisms ?

Going by my repair experience (I was more into TV than VCR though) at first there was a large variety of sometimes quite complex but fascinating mechanisms.
While closening to their extinction time there where a handful of simplified to the bare minimum flimsy designs by LG/Philips, Samsung, Panasonic and a couple others that mostly felt the same low-end junk.

[CaptDon]

I have seen also fluid clutches. they were clear plastic and had a very thick fluid inside. Then there are 3-motor units with no clutch. Sometimes there is an idler wheel pulled or pushed by a spring against the takeup hub and allowed to slip as needed. There has to be 5 or more prevalent designs in the marketplace.

[amyk]

Over the years, how similar were tape players to each other,  and likewise with VCR mechanisms, did most companies make them about the same ? Or was it a variety show of mechanisms ?

VCRs varied quite a bit, although there were far more brands than OEMs. I believe the earliest VCRs used a belt-drive capstan and a rubber idler that slipped (with a tensioning spring that moved to set the torque for playback vs rewind/fast forward), and over time they started using direct-drive capstan motors and a geared idler + clutch arrangement (with a mechanism to lock the clutch for rewind/fast forward). Some older high-end models used direct-drive reel motors, one for supply and one for takeup, but those weren't common. The last generation mechanisms all converged on a 3-motor (drum, capstan, loading) design with geared idler + clutch, and excepting over-cost-optimising that reduced the strength of the parts, can be very reliable due to their simplicity.

[wasedadoc]

I had (and still have in its box in the loft) one of the first Mitsubishi VHS recorders.  A heavy top-loading beast.  The IR remote kit (remote control and external IR sensor which plugged into the main body) was an optional extra.  5 motors - tape extraction/threading, head drum, capstan, supply reel and take-up reel.

I don't remember the year I bought it but I do remember that E180 tapes were £8 each.

[Jeff eelcr]

Most cassette decks have about 40 Grams of tension in take up.
Jeff

[Xena E]

In answer to the OP.

Many of the clutch mechs., have an accessible spider spring that engages on steps on the perimeter of the takeup drive pulley, this can be turned to engage a different step to increase or decrease the tension.

Best done with a tape tension gauge as others have said.

[MarkoGVA]

Interesting, but my spring (as seen in pictures above) does not seem to have been intended for a particular placement on the clutch disk? I do not see any wedges, holes, tabs, etc. on which to "hook" it?

[Ian.M]

I see what's probably your problem.  The hub of the clutch hall with the felt has split so I'd bet its now slipping on the shaft.  Dot the shaft with a marker pen and try turning it by hand while holding whatever it drives and I reckon it will spin on the shaft with very little torque.

[inse]

removed

[MarkoGVA]

Actually, that disk (with felt) was pretty well glued to the shaft, so I am not convinced that that is the problem, but might be....I will re-test. So I guess the "crack" is not supposed to be there?

[inse]

No, the crack is the sign of a general problem, Nylon gears (or similar) press-fitted on steel parts very often fail even after only a few years.
Either the plastic turns brittle or shrinks a little bit and the pressure of the shaft will crack it, so it doesn’t have grip anymore.
Recently had to discard a DV camcorder due to multiple plastic parts failure (the clutches were affected there as well)

[MarkoGVA]

So, what's the solution?
Replace the plastic part, but where to find a replacement?

[Xena E]

Interesting, but my spring (as seen in pictures above) does not seem to have been intended for a particular placement on the clutch disk? I do not see any wedges, holes, tabs, etc. on which to "hook" it?

There quite clearly is.

It seems from your picture that the metal part with the two tabs has been bent and should be flat.

The spring sits against that and depending what step on the plastic part those metal tabs sit against/engage in provides adjustment of the back tension of the take up spool.

How it is shown in the photos it would stretch the tape.

Someone has ignorantly bent the tabs inward trying to adjust the clutch.

This is quite apart from any other problems that may be present.

For what reason did you want to investigate taking the machine apart in the first instance?

[MarkoGVA]

Wow! Thanks for this information. It explains everything (or at least a lot of things  )
I did not bend it. I sent it to the technician some years ago and he said he could not do better.
The problem at that time was small amount of wow and flutter. After service, the technician said that I should use good quality tapes.
Then I did not use the deck for some years. Now, I decided to put it back into service, but by this time the belts have disintegrated so I changed them. Still having a small amount of wow and flutter.
The playback frequency output of 3000 Hz test tape shows variation between 2850 and 3005 Hz. I would set it more precisely with the motor screw, except that it keeps changing-varying. It is not stable. So, my conclusion was that perhaps the instability is due to the clutch being "too weak" not taking up the tape steadily. Therefore, increasing the tension, for example by moving the metal part to the higher slot would solve the problem?
The belts are new, idlers have been cleaned and lubricated, move smoothly. I see no other reason for the wow and flutter.
But I could be wrong.
Any ideas would be appreciated.

[Ian.M]

Know anyone with a lathe?   Fortunately the cracked part is the one with the felt, so a replacement could be machined from brass and secured to the shaft with a couple of set screws.

OTOH if the previous repairer succeeded in bonding the cracked clutch half to the shaft, so it isn't slipping, maybe leave well enough alone.  The speed instability may be a motor fault, or gummed up oil in various bearings or a pinch roller that's taken a permanent set so needs to be replaced.

[MarkoGVA]

How could I test if the motor is turning steady?

[Ian.M]

Print a strobe tacho disc with about 20 black sectors, stick it on the end of the shaft or face of the gear or wheel you want to check the speed of,  and illuminate with a strobe adjusted till the sectors appear to stand still.  If the speed is fluctuating you wont be able to get them to stay still or if the fluctuation is too fast to see, they'll be blurry. 

A slice of a cork with a hole drilled to fit can help attach the disc to shafts, small pullies etc.  Even a DIY NE555 LED strobe will do, just keep the duty cycle low and the LED current high so the flash is nice and sharp to minimise motion blur . . .

[MarkoGVA]

A technical question....
Which one of these parts are spinning and which are stationary?
Also, should the left-most end part (the one that makes contact with the play idler) have some clearance from the case, or should it fit "snugly"?

P.S. As you can see, I am still struggling with the clutch. I straightened the metal piece which was bent, and glued the end cap, but don't know if this will be enough. The clutch is now worse....not picking up the tape at all.