Worm gear pairs typically consist of bronze or brass (or cast iron) worms mated with steel worms. The worm gear transmission offers advantages such as compact size, high reduction ratios, smooth operation, low noise, and high output torque. The meshing mode is predominantly sliding friction, with high contact pressure, high sliding speed, and accompanied by scuffing, making it extremely difficult to form an oil wedge. Therefore, worm gear oils require not only good oiliness (low friction coefficient), viscosity-temperature characteristics, and thermal oxidation stability, but also excellent extreme pressure (EP) anti-wear properties and superior rust and corrosion protection. Borate EP anti-wear agents possess these combined characteristics.
Synthesis and Properties of Borate EP Anti-Wear Agents
In my research, I have focused on the synthesis of borate additives using a “potassium hydroxide–water solution method” involving high-base-number calcium petroleum sulfonate, polyisobutylene succinimide, and boric acid. The reaction mechanism utilizes the alkaline components in the high-base-number sulfonate to react with boric acid, forming calcium tetraborate:
$$
4\mathrm{H_3BO_3} + \mathrm{CaCO_3} \longrightarrow \mathrm{CaB_4O_7} + \mathrm{CO_2} + 6\mathrm{H_2O}
$$
This calcium tetraborate then reacts with a potassium hydroxide aqueous solution to produce a mixture of potassium metaborate and calcium metaborate:
$$
\mathrm{CaB_4O_7} + 2\mathrm{KOH} \longrightarrow 2\mathrm{KBO_2} + \mathrm{Ca(BO_2)_2} + \mathrm{H_2O}
$$
After centrifugal separation to remove larger particle sediment, the fine amorphous borate particles are uniformly dispersed in the oil with the aid of dispersants, forming a stable colloidal solution.
The borate additive exhibits remarkable extreme-pressure and anti-wear performance. When added to a base oil at a boron content of 0.6% (unless otherwise stated), its load-carrying capacity surpasses that of S-P type and Pb-S type oils. The film strength of the boron-containing oil is three times that of Pb-S type oil and twice that of S-P type oil. Furthermore, borates demonstrate excellent oxidation stability, as verified by various oxidation tests. The application results in Table 1 indicate that the service life of borate lubricants is four times longer than that of conventional S-P type lubricants.
| Parameter | S-P Type Oil | Boron-Containing Oil |
|---|---|---|
| Copper oxidant weight loss / % | 5.700 | 0.018 |
| Benzene insolubles / % | 0.34 | 0.15 |
| V100°C increase / % | 14.8 | 4.9 |
In addition, borates have excellent corrosion and rust resistance — no corrosive effects are observed on copper at high temperatures or on steel at low temperatures.
The anti-wear mechanism of borate EP additives differs from classical theories. Two main viewpoints prevail: the deposition film theory and the boron diffusion theory. I tend to favor the J.H. Adams view. Because borates are actually colloidal dispersions of fine particles in mineral oil, transmission electron microscopy (at 96,000× magnification) confirms that the borate particles are amorphous microspheres with an average diameter less than 0.1 μm. These particles physically adsorb onto sliding surfaces through electrostatic charges generated during motion, forming an elastic deposited film. Research by Zhengzhou Mechanical Research Institute has shown that on nitrided and carburized hardened tooth surfaces, a non-reactive deposited film is formed preferentially in the meshing contact areas, and this film is neither uniform nor continuous.
Application of Borates in Worm Gear Oils
Using HV120BS and HVI650 base oils produced by solvent deasphalting, solvent refining, solvent dewaxing, and clay finishing processes, I compounded the borate additive into worm gear oils. The base formulation immediately showed excellent EP properties, good corrosion resistance, and oxidation stability. The key challenges were selecting the proper oiliness agent and resolving the poor anti-foaming performance.
Selection of Oiliness Agents
Industrial worm gears typically consist of hardened steel worms mated with phosphor bronze worm wheels. With high reduction ratios and high load-carrying capacity, the worm gear motion is primarily sliding and scuffing. Therefore, the lubricant must have good wetting and adhesion to the tooth surfaces as well as low friction coefficient. The choice of oiliness agent is critical. I evaluated several commercial oiliness agents along with an organic boron compound, a phosphate ester, a fatty acid ester, and a fatty acid epoxy ester synthesized by our research institute. The results are presented in Table 2.
| Sample | Four-ball wear scar d29260 min / mm | Friction coefficient (Terry oiliness tester) f | Friction coefficient (HQ-1 tester) μ |
|---|---|---|---|
| Base oil | 0.832 | 0.128 | – |
| Oiliness agent 1 | 0.728 | 0.112 | 0.143 |
| Oiliness agent 2 | 0.399 | 0.108 | – |
| Oiliness agent 3 | 0.364 | 0.100 | 0.099 |
| Oiliness agent 4 | 0.624 | 0.110 | 0.184 |
| Oiliness agent 5 | 0.364 | 0.102 | 0.037 |
| Oiliness agent 6 | 0.728 | 0.108 | – |
Note: f is the friction coefficient from the Terry oiliness tester; μ is from the HQ-1 tester with a copper-steel friction pair.
Because the borate additive has a high base number, oiliness agents with strong acidity may cause precipitation. Therefore, I selected agents with low neutralization numbers. Tests showed that organic boron compounds and phosphate esters provide good steel-steel friction reduction, but their copper-steel friction reduction is average. Oiliness agent 3 and agent 5 exhibited excellent performance on copper-steel friction pairs. Moreover, agent 3 showed a synergistic effect with the anti-foam additive. Ultimately, oiliness agent 3 was chosen for the final formulation.
Investigation of Anti-Foaming Performance
The borate EP additive manufacturing process involves both ionic and non-ionic surfactants. These large amounts of polar substances reduce the liquid surface tension and form robust liquid films surrounding gas bubbles, making foaming easy. Meanwhile, these surfactants adsorb onto the bubble film surfaces, inhibiting film thinning and stabilizing the foam. Consequently, adding borates to the oil tends to cause persistent foam. In practice, excessive foam in worm gear reducers can lead to oil overflow and mechanical failure. I tackled this issue by incorporating anti-foam agents T901 and T902, both alone and in combination, and even using colloidal milling dispersion, but none achieved satisfactory results.
However, I discovered that compounding with the selected oiliness agent effectively solved the foaming problem. Adding agent 5 had little effect, but combining agent 3 with the borate additive significantly improved anti-foaming. The probable mechanism is that the unsaturated olefins in agent 3 adsorb some of the surface-active substances, increasing the liquid surface tension, and together with the anti-foam inhibitor, the oil becomes less prone to foaming and the foam collapses more readily. Table 3 summarizes the anti-foaming test results.
| Additive System | Anti-Foam (×10−6) | Addition Method | Foaming tendency (mL/mL) 24°C – 93°C – after 24°C |
|---|---|---|---|
| Borate composite main agent | T901 50 | A | 560/450 – 600/200 – 500/400 |
| T901 50 | B | 500/350 – 550/25 – 450/300 | |
| T902 50 | A | 500/370 – 290/0 – 450/30 | |
| T902 50 + T901 30 | A | 430/300 – 80/20 – 290/210 | |
| Borate main agent + oiliness agent 5 (0.5%) | T901 50 | A | 420/300 – 600/130 – 500/360 |
| T901 50 | B | 350/250 – 430/10 – 470/380 | |
| T902 50 | A | 350/250 – 500/120 – 470/300 | |
| T902 50 + T901 30 | A | 380/295 – 460/0 – 480/320 | |
| Borate main agent + oiliness agent 3 | T901 50 | A | 90/50 – 190/0 – 70/30 |
Note: A – anti-foam diluted with kerosene and added directly; B – dispersed by colloidal mill.
Full Quality Evaluation of Worm Gear Oils
Through formulation screening, I compounded the borate additive with other functional additives to produce EP worm gear oils of viscosity grades N220 and N320. The physical and chemical properties were evaluated, and the oils underwent the WL-100 worm gear bench test at Zhengzhou Mechanical Research Institute. Table 4 presents the properties and benchmark results. All indices and bench test results met the contract specifications, and the products passed the technical appraisal of the China Petrochemical Corporation in December 1995.
| Parameter | MIL-L-18486B(OS) Specification | Contract Specification N220 | Contract Specification N320 | Result N220 | Result N320 | Test Method |
|---|---|---|---|---|---|---|
| V40°C / mm²·s⁻¹ | 198–242 / 288–352 | 198–242 | 288–352 | 218.7 | 314.7 | GB/T 265 |
| Viscosity index, ≥ | 120 | 100 | 100 | 103 | 105 | GB/T 2541 |
| Pour point, ≤ / °C | −12.2 | −12.2 | −12.2 | −13 | −15.7 | GB/T 3535 |
| Flash point, ≥ / °C | 177 | 180 | 180 | 274 | 295.9 | GB/T 267 |
| Fire point / °C | 210 | report | report | — | — | GB/T 267 |
| Sulfur, ≤ / % | 1.25 | 1.25 | 1.25 | 0.45 | 0.76 | GB/T 267 |
| Water, ≤ / % | 0.0 | trace | trace | none | none | GB/T 260 |
| Neutralization number, ≤ / mgKOH·g⁻¹ | 0.30 | 0.30 | 0.30 | 0.04 | 0.03 | GB/T 4945 |
| Saponification number, ≤ / mgKOH·g⁻¹ | 25.0 | 25.0 | 25.0 | 12.21 | 10.31 | GB/T 8021 |
| Chlorine / % | 0.0 | — | — | — | — | — |
| Copper strip corrosion (100°C, ≤) | slight tarnish or darkening | 1 grade | 1 grade | 1a | 1a | GB/T 5096 |
| Rust prevention (D665B) | no rust | no rust | no rust | none | none | — |
| Four-ball weld load (N, ≥) | 392 | 392 | 392 | 418 | 447 | GB/T 3142 |
| Anti-foam (mL/mL after 10 min) | GB/T 12579 | |||||
| 24°C | 300 | 100 | 100 | 40 | 50 | |
| 93°C max | 25 | 25 | 25 | 15 | 0 | |
| after 24°C | 300 | 100 | 100 | 80 | 30 | |
| WL-100 worm gear bench test | Bench test | |||||
| Efficiency, ≥ / % | — | 74.0 | 73.0 | — | 73.5 | |
| Scuffing load, ≥ / N·m | — | 680 | 680 | — | 770 |
Borate is a highly effective, multipurpose, non-toxic, and odorless lubricant additive. It possesses unique EP, anti-wear, and friction-reducing properties, outstanding oxidation stability, corrosion and rust protection, and good seal compatibility. It has been successfully applied in automotive gear oils, industrial gear oils, rust-preventive dual-purpose oils, and machine oils, demonstrating significant energy-saving effects.
Because of these special properties, borates also perform excellently in worm gear oils. I formulated N220, N320, and N460 boron-containing EP worm gear oils, which have been widely used in southern China, primarily in elevator traction machines. Long-term field applications have confirmed that boron-containing EP worm gear oils offer high load-carrying capacity, good rust and corrosion protection, excellent thermal stability, low noise, and low temperature rise.
Practical Application of Boron-Containing Worm Gear Oil
In January 1995, I conducted a field trial of the N320 boron-containing EP worm gear oil in the rear axle worm gear reducer of a Q20 trailer at Zhanjiang Port Authority. The results showed that the oil possessed strong anti-wear and anti-scuffing properties, high load-carrying capacity, high transmission efficiency, good rust and corrosion resistance, excellent thermal-oxidative stability, low noise, and low temperature rise. No oil overflow or mechanical failure due to excessive foaming occurred during operation. After more than one year (approximately 3,600 hours of service), the reducer was disassembled and inspected. The used oil exhibited no significant changes in physical or chemical properties compared to fresh oil. The worm gear tooth surfaces were clean, with no signs of rust, corrosion, pitting, fatigue wear, or scuffing. The oil tank had no deposits, and the worm gear pair could continue to be used.
Despite the superior performance, borate additives have one serious drawback. When a large amount of water enters a borate-containing lubrication system, the borate crystallizes out of the oil into hard particles, increasing friction. Tests have shown that when the water content is less than 1%, there is no significant effect; between 1% and 3%, the oil can still be used; but when the water content exceeds 5%, the additive fails. Similarly, if the manufacturing process does not strictly control conditions, resulting in excessively large borate particles, friction will also increase — this was confirmed by the WL-100 worm gear bench test results. To improve the water resistance of borates, one must study the type and ratio of dispersants or compound borates with other types of EP agents.
Conclusions
- The synthesis process of borate additives is mature, and their properties are unique; their mechanism of action differs from classical theories.
- The EP worm gear oils formulated with borate as the main additive meet all physical, chemical, and bench test specifications.
- Borate is a new type of EP anti-wear agent with a wide range of applications, showing excellent performance in worm gear oils.
- Boron-containing EP worm gear oils satisfy the lubrication requirements of worm gear reducers, improving equipment service life and operational reliability. These oils are particularly suitable for sealed, heavy-duty worm gear reducers.