Why does a glazed or hardened pinch roller cause trouble??

On the subject of the capstan drive, I am curious about something.

On most of my machines, including a Studer A807 and Otari MX80, the pinch roller presses against the backing, with the oxide surface pressing against the capstan. However, on the MSR-24, the oxide presses into the pinch roller, with the capstan shaft pressing against the backing.
Is there an advantage to either configuration?
 
Interesting. Both types of large format machines I have have the pinch roller on the oxide side.

Imagine if the pinch roller was the same hardness as the capstan. Im pretty sure the performance wouldn't be, uh, great.

My understanding is that the pinch roller needs to be soft enough to A) get a small amount of wrap around the capstan. This gets us from LINE contact (zero area) on the capstan, as two hard rollers would provide, to AREA of contact, which is required to drive the tape, and B) be supple enough to make up for the small gap caused by the thickness of the tape, at least on machines with rollers wider than the tape to aid in driving the pinch roller. Either way it is AREA of contact that relates to quality of drive. It is AREA times PRESSURE times CO-EFFICIENT of FRICTION that determines strength of the drive. So as a roller hardens over time the AREA of contact is reduced and performance suffers accordingly.
 
This ^^^^ is my understanding as well.

I think a majority of "semi-pro" to pro machines have the capstan on the backing of the tape, not the oxide, which makes perfect sense. I wouldn't want the hard metal or ceramic surface of the capstan in contact with the oxide as it is getting the tape up to speed when engaged. There is always some momentary slip.
 
Interesting reading. :)

My layman's understanding of this all is a bit simplistic.

A hardened, glazed pinch roller is like driving on bald tires in the rain.
:D
 
My bad, the Otari does have the oxide against the pinch roller. So it looks like a large-format thing...
 
It's not a large format thing.

Out of the 9 tape machine I have either owned or worked on, only one has the oxide facing the capstan, and you can see I've touched a mix of 1/4" consumer-format machines and 1/4" to 2" pro-sumer (i.e. narrow track format multitrack) to professional format (wide track multitrack or 1/4" halftrack).

Here are the eight with the capstan against the tape backing:

1/4" consumer Pioneer RT-909
1/4" consumer Sony TC-630
1/4" pro Tascam BR-20T
1/4"-1" pro Ampex AG-440
1"-2" pro Ampex MM-1000
1/4"-1/2" pro 3M M56/64
1/2" prosumer Tascam 58
1/2" prosumer Tascam 48

Here's the one with the oxide against the capstan:

1/4" prosumer Tascam 388
 
(from: Why doesn't friction depend on surface area?)

Question
Why doesn't friction depend on surface area?
Asked by: Elizabeth Stewart

Answer
Although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together.

If you were to increase the force as you increased the area to keep PRESSURE the same, then increasing the area WOULD increase the frictional force between the two surfaces.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
Click to expand...
ie, the strength of the pinch roller spring and the material it is made of in contact determines the friction between it and the metal capstan. Somehow you seem to lack this knowledge but I'll give you the benefit of the doubt.
 
sweetbeats said:
It's not a large format thing.
Out of the 9 tape machine I have either owned or worked on, only one has the oxide facing the capstan, and you can see I've touched a mix of 1/4" consumer-format machines and 1/4" to 2" pro-sumer (i.e. narrow track format multitrack) to professional format (wide track multitrack or 1/4" halftrack).
Click to expand...

Actually, it's starting to look like it's a European thing, Tascam's prosumer decks notwithstanding.

It's not just the A807, which admittedly was Studer's budget deck, but if you look at the A80, even the A827, all these machines are capstan-to-oxide.
The Telefunken M15 is the same, likewise all Lyrec machines (except for the FRED which is capstanless). Nagra as well. Even more obscure European decks like the Mechlabor STMs work that way.

And that makes me start to wonder if there was some kind of patent-dodging trickery going on.
 
Just to throw a spanner into the conversation? I have just remembered that the first Ferrograph decks, the Wright&Wear 'battleships' used a solid BRASS pinch roller and the capstan had a thin clear plastic sleeve. Thus it was the capstan that did the friction drive. Seems like a system prone to wear and failure but in fact they didn't. The W&W deck was known for its bomb proof reliability. It was however not THE best deck for wow and flutter.

Dave.
 
"Is there an advantage to either configuration"?

Probably not. But, parts inventory and sourcing might
 
garww said:
"Is there an advantage to either configuration"?

Probably not. But, parts inventory and sourcing might
Click to expand...

I seem to recall that state of art tape machine began using ceramic capstans? These would give good 'grip'. I also remember that rubber was replaced with polyurethane, same stuff high performance steering bushes are made from and is bloody nearly indestructible.

Shame innit? The materials and digital control science came jeeeust too late to save the tape recorder!

Dave.
 
Well, I don't have anything greater than 1/4-inch and I don't know if this is much of a challenge 'til we add mass and speed to the spools. I'd guess my largest capstans are about 7/16-inch
 
Come on guys - high school physics stuff. It's simply friction. A shiny polished surface that is hard has very little friction. A rubber roller has 'give' in it, which increases the surface area of the pressure roller onto the tape surface. A hard roller has little friction, and a soft rubber one has lots - as my engineering tutor used to say, it's not friction, it's stiction. The pressure of that pinch needs to be just right, and as somebody else said, the caption/pressure roller combination drives the tape through the machine, and the spools simply have enough take up pressure to wind neatly - they play NO part in pulling the tape past the heads.

Soft rubbery stuff is good. Hard rubbery stuff is bad - and the imperfections of an old wheel do indeed pull the tape through unevenly - anything less than 90 degrees to the head azimuth is bad, and does, as said above mess up tracking!
 
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No, many capstan are not overly finished and I've hard rubber that works fine, But it is still the process you describe. This early '60s capstan picks up oxide easily
 

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