Sliding contact
Worm gear oil viscosity
A worm and wheel slide. Spur and helical teeth roll through the mesh with a short slide at the ends of the contact. The slide in a worm gear is the contact. It wants a thicker oil than a spur set at the same shaft speed, and on a bronze wheel it wants a lubricant that will live with copper alloy.
AGMA 9005-F16 keeps wormgears on their own recommendation for that reason. Use the wormgear figure in the standard, not the spur and helical figure, and not the orientation bands on the industrial gearbox page. Those bands are not the worm chart. The chart itself is not copied here.
Grades you will see on a plate
ISO VG 460, 680, and 1000 are the grades that show up on cylindrical worm boxes. A small, fast, cool-running worm may be specified lighter. A slow, hot, heavily loaded worm may be specified heavier. The 40°C window is on the ISO VG chart. The running viscosity is the column that matches the sump, not the name of the grade.
| °C | 460 · VI 95 | 460 · VI 150 | 680 · VI 95 | 680 · VI 150 |
|---|---|---|---|---|
| 20 | 2,054 | 1,540 | 3,260 | 2,406 |
| 40 | 460 | 460 | 680 | 680 |
| 60 | 147 | 177 | 206 | 250 |
| 80 | 61.0 | 82.5 | 81.4 | 112 |
| 100 | 30.3 | 44.2 | 39.1 | 58.2 |
Same assumption as the other temperature tables: midpoint at 40°C, viscosity index by ASTM D2270, other temperatures by ASTM D341. Your data sheet replaces it.
Compounded mineral oil and PAG
Steel on bronze is the usual materials pair. Mineral oils for that pair are often compounded: a gear oil with a fatty additive so the bronze has a boundary film under the slide. AGMA 9005 is where the compounded-oil description and the approved use live. Polyglycol (PAG) worm gear oils are the other common answer. They usually run cooler and waste less power, because the viscosity index is high and the friction in the slide is lower.
PAG and mineral oil do not mix. Changing chemistry means a drain and a flush, and the seals, the paints, and any aluminum in the system have to be checked against the new oil. A hotter box is not automatically cured by pouring a heavier mineral grade on top of a tired PAG, or the other way around.
Pitch diameter and ratio, if you are still identifying the set, are on gearcalculators.com. Gear engineering notes are on evolventdesign.com. Branded oil charts are on oilviscositychart.com.
Get a lubricant recommendation for your gearbox
Send the gearbox type, the oil temperature it actually runs at, and what is in it now. A gear engineer at Evolvent Design can point you at a viscosity grade. The form is not live yet: it posts to a placeholder address in the site config, and nothing is stored.
Questions
Can I use the same EP gear oil in a bronze worm wheel?
Not because it worked in a helical reducer. Some extreme-pressure sulfur-phosphorus additives are hard on bronze. Worm plates that call for a compounded oil or a PAG are specifying that chemistry. Match the plate, or ask before you substitute an EP oil from the spur-gear shelf.
The worm box runs hot. Should I go up a grade?
First find the viscosity at the temperature it has reached. A heavy grade that is churning can be the heat. AGMA 9005’s wormgear recommendation, the OEM plate, and a look at mineral versus PAG are the decision. Climbing one ISO grade without that check often adds startup loss and not much film at the new, higher temperature.