"There is a theory that polyalkylene glycols (PAGs) perform better in worm gears than other lubricants due to the lubricity factor involved. Is this true and what benefits do they offer over polyalphaolefin (PAO) worm gear lubricants or mineral oils properly additized for worm gears?"
There are a few reasons why a properly formulated PAG might perform better in specific circumstances than an equally properly formulated mineral oil in a sliding contact zone.
Gear oils based on glycol stocks are highly polar. The extra surface affinity provides low frictional coefficients without the use of additives. This could be referred to as "lubricity."
Once fortified with the right complex of additives, glycol lubricants can provide exceptional load-bearing performance (film strength).
Glycols can also have a superior pressure-viscosity coefficient, which refers to the relationship between the load placed on the oil film (pressure) at the dynamic load zone and the thickness of the oil film (viscosity) at that load, when all other factors (material, temperature, geometry, speed, load) are constant.
The pressure-viscosity coefficient gives us fixed values for lubricant film thickness in a given set of conditions (elastohydrodynamic regime, also known as an EHL or EHD regime), based on a mathematical estimation. The actual unit of measure (mm2/N) is less useful than the percentage improvement of the synthetics over the mineral oil at the given temperature points as noted in the information sheet.
For example, at a temperature of less than 80 degrees C, mineral oil provides thicker films than a PAO lubricant, and at a temperature of less than 57 degrees C, mineral oil offers thicker films than a PAG lubricant. In the temperature range of 70 to 90 degrees C, there is only a 5-percent difference between the EHL film thickness of mineral and PAO lubricants. In this same temperature range, a PAG lubricant gives thicker films ranging from 16 percent to 37 percent thicker than mineral oil.
Obviously, thicker EHL film formation contributes to reduced risk of wear and better long-term reliability.
It is worth noting that the performance of a lubricant in any given application depends on a number of factors, including performance expectations, machine application, machine design and operating environment. While a PAG might provide superior overall performance in certain cases compared to mineral oil or PAO, there are other cases where a non-PAG lubricant in a worm gear application may be the optimum choice.