The Talking Machine Forum — For All Antique Phonographs & Recordings

Years ago I remember reading about the pounds per square inch at the tip of a new steel needle in an Exhibition sound box when it is placed on a record. Does anyone have a figure?

Thanks, Jerry Blais

I don't know about the pounds per square inch, but here are the tracking weights:

Victor Exhibition: 100 grams
Victrola No. 2: 120 grams
Victor Orthophonic: 135 grams

In order to figure this out, we would need to know the surface area of the needle in contact with the record which of course is microscopic. I'm guessing that the pressure would be immense.


The formula:

P = F/A

where

P is the pressure
F is the applied force
A is the surface area where the force is applied

Anyone know the area of the tip of a new needle? Jerry

You can make an estimate of the needle tip's area, and plug it into the formula, and obtain some crazy number that will be impressive...but I guess the real thing to keep in mind is that the pressure is just part of the whole system. So the type of steel, the type of shellac - and the hardnesses of each, and the frictional characteristics of each - all factor in on record wear.

So, yeah, it's a zillion PSI, but the record can handle it, and the needle wears down. Of course, as the needle wears down the contact patch widens and the concentration of pressure lessens...but the new damaging factor of the sharp sides of the needle's growing flat spot comes into play. That's why you dump the needle after a single side is played, and never re-use or rotate a used needle.

I wonder whether the firms that made both records and needles actually did research on the compatibility of the two items, and made their records with a certain level of abrasiveness and their needles with a specific level of . It very well could be that their recommendation to use only XYZ needles on your XYZ records might be more than a sales pitch.

The force is much more complex than described. Two flat and stationary surfaces meeting vertically is a simple equation, but the contact areas here are not flat and the forces are not just vertical. Consider the shape of the groove alone. Because it is a variably curving groove and the needle point is closer to a spherical radius, the contact shape and directional forces are complex and even change as you get closer to the center of the record. When the groove is in motion, the directional forces change as well. This is a simplified description of a dynamic situation.

Static friction is much greater than kinetic, so the needle in motion produces less wear. Letting a record slow down will cause increased friction and wear as the friction increases.

To make a long story over, in a static situation, the needle exerts a highly concentrated downward force. The force that is likely to wear a record is that caused by the sides of the needle, first having to move the reproducer and arm or horn across the record and second, overcoming the mechanical resistance between the needle and diaphragm required to produce the sound waves. I don’t think that the tip of the needle contacts the bottom of the groove anyway, but I may be wrong about that.

Another variable might be whether the machine is a front- or rear-mount; and if the former, how large a horn is on it.

Bill

I'm taking a few machines to a media class at our local high school and wanted a pounds per square inch number to throw at them. It's not a big deal. Jerry

"Assuming the diameter of the needle point bearing surface to be .003 inch, and the needle pressure is 5 oz., the resulting vertical unit pressure is roughly 44,000 lbs. per square inch. The counterbalancing weight may be seen at the right end of the pickup. It is not desirable to completely counterbalance the weight of the pickup head and arm, for some weight must act on the needle in order to keep it from jumping out of the groove when loud low-frequency notes are being played"

Maybe that will help. I just read that a few days ago. Its from a 1932 book called "Radio Physics Course" There's a section on phonographs, pickups and amplifiers. I can scan the page if you want, there's a little more information about the subject. Good luck with it now. Earl

In their advertising circa 1930, BCN (Burmese Colour Needle Co.) claimed 12 tons per square inch (Imperial tons), which is 13.44 US tons per square inch.

Earl H's source above (44000 lbs) is 19.64 Imperial tons or 22 US tons per square inch.

Wow!

I've heard the phrase "Baldwin steam locomotive" used on here in regards to needle/stylus pressure.