Modern Oil for a Vintage Lathe: AA-14 Monarch
Finding modern oils for vintage equipment can be a tricky challenge. At its best, the manual will tell you exactly what you need to buy. At its worst, you have a dry machine with nothing with no manual, just mysteries. We found ourselves somewhere in the middle with the Monarch AA-14 lathe.
Information was limited on the Monarch, and we knew the owner had some troubles getting just the right oil. Too thick (high viscosity) and flowability was an issue, too thin and the oil would would displace and fail to lubricate the machine's ways. Ultimately, we were able to make our recommendation off a combination of converting between standards and pitch line velocity
First thing's first we checked the manual. The original equipment manufacturer (OEM) is the best place to go when looking for information on old equipment. In our case, the OEM has long been out of business, and all we have left is a manual. With no preference to brand, type, or product line, the manual recommended an "SAE30" oil. These have traditionally been used in the automotive space and, as such, they tend to be loaded with detergents. The detergents keep soot and combustion residue out of the engine, but aren't so important in machinery.
Not wanting to get an automotive oil, we opted to see what industrial machinery oils were equivalent to SAE 30. Fortunately, the crossover chart shows that SAE 30 lines up with ISO VG 100. More specifically, ISO 100 is a subset of SAE 30, the range of SAE 30 oil viscosities entirely encompasses the ISO 100 range. This is good to check to make sure that any oil we get within the ISO 100 range is still within the range recommended by the manufacturer.
To check our results we needed a few parameters (and made a few assumptions) about the machine. During operation we knew the maximum machine speed was 1000RPM, and a quick look in the gearbox told us we were dealing with about a 100 tooth gear. We estimated the gear pitch and calculated the pitch line velocity (PLV). From the calculator, we got that: 100 teeth, diametral pitch 40, at 1000RPM, would give a pitch line velocity of 3.3249m/s.
Pitch line velocity in-hand, all we needed was the operating temperature of the machine. A reasonable estimate was 45ºC (113ºF). Looking through the AGMA recommendation charts our operating conditions landed us right at a viscosity of 100 cSt, an excellent confirmation of the first comparison. While the AGMA charts depend on Viscosity Index we determined that a VI of 90 (standard for conventional, non-synthetic oils) was quite reasonable given the lax specifications from the manual and the minor temperature swing the machine would experience.
As a final step, we took the information we had and checked it against our Big List of Oils. Looking for: a conventional oil, ISO 100 viscosity, that's good for Gears, gave us 9 options spanning the four major manufacturers (Shell, Mobil, Chevron, and Castrol). Looking at availability (and what our local distributors carried) we happily recommended Mobilgear 600 XP 100 for the Monarch AA-14 Headstock.
The Mobil Vacuoline 525 is also an excellent choice, but was unavailable through our local suppliers. While Mobil also offers Glygoyle 100, which was another suitable candidate, and as a synthetic oil (not a requirement for this machine) it had a higher Viscosity Index. However, since the Glygoyle 100 is a Food-Grade oil it was prohibitively expensive to go into the lathe headstock and carriage.