In the last post I discussed how I cleaned the interior parts of the TEAC A-4010S tape machine are prepared it for checking the torque at the reels. The problem, as you may recall, was too much tension on the tape causing the thinner tapes to snap.
The image on the right shows how the service manual for the machine explains the method for checking the tension on the tape. Tension is caused by the reels either pulling on the tape or resisting the pull of the tape. It depends on whether the tape is being played, fast forwarded or rewound. The A-4010S adds one complication in that it can play in either direction.
What you see in the photo is quite interesting. The machine has a standard 7″ reel mounted on the left spindle. This reel has a 4″ hub. You can find at least one set of videos on YouTube that show a home-made 4″ hub on an old reel. In this case I used a standard reel from a commercial tape as it was easy to come by. I had on in the collection. Many, in not most, commercial tapes used 4″ hubs.
A string about 30″ long is attached to the reel in the same manner as a tape. The other end of the string is connected to a spring scale, which you can see at the top of the photo. This scale reads up to 100 grams, which is perfect because the optimal load is well under 100 grams.
Following the service manual instructions it possible to obtain a “pull strength” within a certain range by activating the forward and backward buttons. Obviously you have to follow the instructions exactly.
In the case the original values were way too far out of bounds. The force that the scale measured was radically too high, and it was clear that this would cause the tapes to snap.
The scale is measuring the strength of the pull on the string as expressed in grams. This is a function of the torque created by the turning of the reel. When the turning force on the reel is too high, the tension on the tape is so strong that the tape will snap.
Anybody interested in cars will be familiar with engine torque. That is the same twisting force that we see here with the tape recorder. Now, frankly, it is possible to get into a highly technical discussion regarding automotive torque vs horsepower and the relationship between those two values. But, we aren’t talking about cars here. Nevertheless, I got to wondering just exactly how much torque was being applied by the A-4010S.
It seems we are getting into some high school physics here, and that is pretty much where my basic understanding of physics is centered, so I made some calculations and discovered that the torque at 4″ hub on the tape reel is, to be blunt, significantly less than the output of a AA Fuel Dragster. We are not surprised. Automotive torque is expressed at foot-pounds. Torque on the A-4010S can also be expressed in foot-pounds. There is some humor in that.
Below is an illustration of that calculation. Worthwhile, not so much. Interesting, oh, for sure.
Now we have to figure out how to adjust the pull on the tape.