Those cap jewels on train wheels...do they make a difference?

[LittleWatchShop]

I just serviced a Lorsa P72 (looks nearly identical to the P75 to me).

It is a 21 jewel watch where the additional jewels (17+4) are simply cap jewels on the train wheels (third and fourth).

Do they do any good, or just get the jewel count up?

BTW, when assembling the watch, I put the wheels in first (planning to put the cap jewels last).  I think it was the third wheel that was just flopping around...would not seat into the pivot holes.  Well, that is because the design depended on the cap jewel to reduce travel of the pivot through the plate.  Once the cap jewel was in place...all was well.

This was another watch with the MT Antichoc system.  This time, I was ahead of the game and knew how to handle them far better.  What was a three-hour job (the last time) was under thirty minutes...and much less stress.

My journey continues!!

[spectre6000]

Quote

 

A jewel bearing is a bearing which an unlubricated metal shaft spins in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. In operation, the shaft tilts slightly so as to contact the jewel pivot hole at two opposite points. The shaft rolls inside the bearing rather than sliding. As the shaft rolls, the center precesses.

The conical pivot (1) requires two jewels for a bearing, a cap jewel (5) and pierced jewel (6). Unlike the cylindrical pivot, the conical pivot has no "shoulder" and uses the cap jewel to determine end-shake of the wheel pinion (3). This arrangement provides lower friction than the single-jewel cylindrical pivot arrangement. Generally, friction on the conical pivot occurs only at the tip of the pinion on the lower cap jewel (3) or, in a vertical position of the watch (a horizontal position of the pinion) on the thin edges of the holes in the two pierced jewels (6).

The conical pivot is usually used on the balance wheel and, sometimes, on the escape wheel. The balance (and, when provided, escape wheel) anti-shock assembly uses a conical pivot with cap and pierced jewel.

 

From a website I apparently can't link to...

TLDR; It eliminates the friction of(/need for) the shoulder of the pivot shaft from resting on whichever jewel is lower. Also, provides tighter end shake. Also provides hours of fun crawling around on the floor looking for end jewels that pinged off into the ether. Hours! Fun...

Edited July 11, 2021 by spectre6000

[LittleWatchShop]

2 minutes ago, spectre6000 said:

[quote]A jewel bearing is a bearing which an unlubricated metal shaft spins in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. In operation, the shaft tilts slightly so as to contact the jewel pivot hole at two opposite points. The shaft rolls inside the bearing rather than sliding. As the shaft rolls, the center precesses.

The conical pivot (1) requires two jewels for a bearing, a cap jewel (5) and pierced jewel (6). Unlike the cylindrical pivot, the conical pivot has no "shoulder" and uses the cap jewel to determine end-shake of the wheel pinion (3). This arrangement provides lower friction than the single-jewel cylindrical pivot arrangement. Generally, friction on the conical pivot occurs only at the tip of the pinion on the lower cap jewel (3) or, in a vertical position of the watch (a horizontal position of the pinion) on the thin edges of the holes in the two pierced jewels (6).

The conical pivot is usually used on the balance wheel and, sometimes, on the escape wheel. The balance (and, when provided, escape wheel) anti-shock assembly uses a conical pivot with cap and pierced jewel.[\quote]

Very good...I can now see the rationale for doing that.  If the watch were still apart, I would inspect the pivots to confirm the shape...but I guess I will wait for the next P72 to show up.  They seem to have been used buy Waltham and I have another showing up via ebay soon.

[spectre6000]

I also noticed the cap jewels showing up on watches in an apparent attempt to boost jewel count. There are absolutely situations where that is clearly the sum total of the intent and effort (singly jeweled watches for instance). I remember reading that "fully jeweled" means 15 or more jewels. I used to think that meant it wasn't necessary for the center wheel to be jeweled. As I've gotten deeper into all of this, I've noticed that the gear train doesn't always have four wheels, and a three wheeled train would be fully jeweled by my intuitive understanding and only have 15 jewels. 17 is the typical number though, and maybe the unjeweled center wheels are sort of the reverse of the jewel count game; taking advantage of the occasional fully jeweled watch only having 15 jewels. Then the cap jewels on train wheels may be the counter to that counter by getting the number back up to 17 (or more). Cap jewels do provide a benefit, but it looks to be one involving chasing the last percent either in fact or perception.

[LittleWatchShop]

On 7/11/2021 at 8:28 AM, LittleWatchShop said:

This was another watch with the MT Antichoc system.  This time, I was ahead of the game and knew how to handle them far better.  What was a three-hour job (the last time) was under thirty minutes...and much less stress.

Tackled another one without any difficulty...I guess I am getting better.

[rodabod]

There is also an argument that the oil takes longer to dry out when the jewel hole is capped. I don’t have anything to prove this though. 

[nickelsilver]

18 minutes ago, rodabod said:

There is also an argument that the oil takes longer to dry out when the jewel hole is capped. I don’t have anything to prove this though. 

I'm not sure if that's such a big deal with modern oils, but I find that I can put about 3 times the oil in a capped jewel than a regular one. That has to help!

[ifibrin]

Capped jewels on train wheels held by detached plates with screws make endshake adjustment impossible.

[spectre6000]

There are definitely movements out there where that's all they're good for. I just picked up a 21J mystery watch. Opened the case to find some of those plated cap jewels on a pin lever movement. Would have been a bummer save for the 404 price. Very interesting watch.

[VWatchie]

On 7/11/2021 at 3:28 PM, LittleWatchShop said:

BTW, when assembling the watch, I put the wheels in first (planning to put the cap jewels last).  I think it was the third wheel that was just flopping around...would not seat into the pivot holes.  Well, that is because the design depended on the cap jewel to reduce travel of the pivot through the plate.  Once the cap jewel was in place...all was well.

I made this mistake myself and was completely dumbfounded until I finally managed to figure it out. So, valuable tip, worth mentioning more than once!

[luiazazrambo]

A similar sort of topic was recently discussed and the pdf I attached is from there. It explains the difference among different types of jewels very well. It is worth reading.

Joseph Bulova School of Watch Making - Unit 9b.pdf