Vintage DD turntables. Are we living dangerously?

The linear temperature coefficient of expansion of Aluminium is 0.000023m/m degrees C and is indeed approx. double that of steel.
Using this figure on a LO7D and assuming a 5 degree C delta, we get a change in distance to spindle of around 0.04mm. (approx 1.6 thou inches) Further the arm is lengthening and shortening relative to the ambient temp. This would somewhat mitigate the distance change.

Of course if one goes the pod way, the shelf's thermal expansion, contraction becomes the decider of the dimensional change.

The thermal expansion "problem" has been grossly overstated.

Richardkrebs
The thermal expansion "problem" has been grossly overstated.

So what are we left with,the stylus moving a 4 or 5lb pod?

Time to exit this carousel.

Totem.
I first posted this on another thread.
Here updated for relevance....

To me it is obvious that a pod will move due to Stylus drag. The question is how much.
In order to calculate this I needed a figure for stylus drag. A search on the web proved inconclusive but then ironically the answer came from the original timeline thread. There, one TT is mentioned where specific data is given on the amount of laser pointer movement per revolution and its distance from the centre spindle.
This TT is a beautifully engineered machine with, from memory a 22 kg platter driven by a fractional horse power motor via a thread. Hereafter I will call this TT. "TD"

The specifics were 2 mm movement on a radius of 400 mm, per revolution.
With this information it is possible to calculate the retardation torque and hence the drag. From this it is possible to calculate how much the pod moves.

So assumptions......
A pod similar to Halcro's is used on TD
Platter 22 kg of uniform section
Pod plus tone arm 11.5 kg
Height to record surface above mounting surface 150 mm
Pod feet 100 mm spacing in a equilateral triangle
Pod/ arm CofG, 75 mm above mounting surface
Pod CofG Central inside the mounting feet
Pod feet are not adhered to the mounting surface. ( no penetration of the cones into the shelf )
The same arm and cartridge used on TD is used on the pod.
TDs motor only provides enough torque to maintain original speed before stylus is lowered, after it is lowered.
Stylus is lowered at a radius of 140mm
Platter has a diameter of 320mm

The first answer is the force applied to the platter to cause this retardation torque. This works out at around 0.0031 newtons. Actually a large number under the circumstances but it is slowing a 22 kg platter!
Using this force and applying it at a height of 150 mm to the pod we get a tilting of 8.2 microns towards the platter, more or less.

Observations.
With the stylus at a radius of 140 mm, the two front feet of the pod do not appear to be a right angles to the arm when viewed from above. This will reduce the tilt a little. It is unknown what happens to the magnitude of the stylus drag as the arm tracks towards the centre, so it is possible that the pod will tilt as calculated once the arm is at right angles to the feet, assuming that this happens before the end of the inner grooves.
As an aside the tilting at a radius of 140 mm produces a yawing effect on the pod such that the arm rotates approximately along its axis. This effect is caused by the configuration of the feet. It is tiny and likely insignificant.
The calculations assume that TDs motor does not sense the slow down and produce restorative torque. Since it is a synchronous motor it will act to try and maintain speed. This will put more energy into the system and increase the tilt.
The pods appear to be slightly crescent shaped. If this is the case the CofG will be biased towards the two feet closest to the platter. This will increase the tilt.

The CofG is probably higher than shown here due to the feet and the weight of the arm. If it is the tilt will be greater.

The calculations assume that TDs platter has a uniform section. If its radius of gyration is larger or smaller than this suggests, the tilt will similarly be larger or smaller.

If TDs arm and cartridge was fitted to a pod and used on Halcro's TT, things would be different again. This because the TT-101 does NOT slow down. It is putting even more energy into the system, so the tilt would be larger.

The calcs assume that the motor assy cannot move. In Halcro's case it can and will.

The 8.2 micron figure is an average. The pod will move more or less depending upon the groove modulation.

Actual dimensions and weight of the pod will materially change these numbers.

I do not know if the amount of movement is of any significance, but yeah baby, it moves with the grooves. This compromises one of the three ideals I mentioned featured in the mythical perfect TT. This was my starting point in these discussions.

So what are we left with,the stylus moving a 4 or 5lb pod?

Ooh no Totem...
The stylus actually moves a 25lb pod...😝💩
And Lewm apparently agrees with this 'theory'....
But here's the really puzzling thing....we have many tools available to actually prove whether or not the pod is actually moving under stylus drag.
Accelerometers, lasers, high-speed cameras etc which any competent scientist would utilise before making a fool of himself.
I have video evidence of the Timeline in action with 3 pods and 3 styli in contact with zero movement displayed yet Richard Krebs prefers to proffer theoretical calculations masquerading as quasi scientific proof intended to add verisimilitude to an otherwise bald and unconvincing narrative.
Give us one shred of practical evidence Mr Krebs....
Just one shred of verifiable evidence instead of your constant and unprovable speculation....

Halcro.

You accept without question that a stylus can slow a 22kg platter. This, while said platter is being driven by a substantial motor. Why can't you accept that a lighter structure can be rocked on its feet by the SAME applied force, which now has a mechanical advantage?

I am not saying that the movement is a problem. What I'm saying is that it exits. It is clear that many people get brilliant results with this type of setup. I myself have auditioned rigs like this and liked the results. But that does not detract from the facts. The pod moves

Check my math if you like, the numbers are sound.