In my work on the carbonate reaction kettle at a pharmaceutical plant, I encountered a persistent leakage problem with the M20 worm gears reducer. This type of reducer is often custom-built in small batches, resulting in low standardization. The original seal design was a packing seal, which required frequent adjustment and replacement of the packing material to maintain elasticity. The limited space between the reducer housing and the coupling—only 200 mm vertically—made replacement particularly difficult. After each replacement, the packing quickly hardened, and lubricating oil from the gearbox began leaking along the shaft. The situation was both inefficient and costly.
Analysis of Original Packing Seal Failure in Worm Gears
The original packing seal configuration is shown schematically in the following diagram. The leakage path was identified along the shaft surface due to the loss of packing elasticity and inadequate radial pressure.
The main reasons for the packing seal leakage in these worm gears were:
- Packing material (usually braided fiber) hardened over time due to heat and oil exposure, losing its sealing ability.
- The shaft radial runout caused by the connected agitator introduced uneven contact, accelerating wear.
- The low rotational speed (6 r/min) of the worm gears output shaft did not generate sufficient hydrodynamic lubrication to protect the packing.
- Frequent manual adjustments were required, but the cramped installation space made proper maintenance nearly impossible.
To quantify the sealing performance, the leakage rate $Q$ for a packing seal can be approximately expressed as:
$$
Q = \frac{\pi d h^3 \Delta p}{12 \mu L}
$$
where:
- $d$ = shaft diameter (155 mm)
- $h$ = average gap between shaft and packing (μm)
- $\Delta p$ = pressure difference across the seal (Pa)
- $\mu$ = dynamic viscosity of the oil (Pa·s)
- $L$ = axial length of the packing (mm)
For the M20 worm gears reducer, the gap $h$ increased rapidly after packing ageing, leading directly to higher leakage.
Attempt to Use Mechanical Seals for Worm Gears
As a first improvement, I considered replacing the packing seal with a mechanical seal. Since the output shaft diameter was 155 mm (non-standard), no off-the-shelf mechanical seal was available. I designed a custom single-end, small-spring mechanical seal for this worm gears application. The structure is shown in the schematic below (conceptually).
However, the mechanical seal also failed to stop leakage. I identified three main reasons:
- The extremely low shaft speed (6 r/min) of the worm gears could not generate a stable hydrodynamic pressure film between the rotating and stationary faces. The minimum required sliding velocity for a reliable fluid film is typically above 0.5–1 m/s, while the actual value was only:
$$
v = \frac{\pi d n}{60} = \frac{\pi \times 0.155 \times 6}{60} \approx 0.0487\ \text{m/s}
$$
This is far below the threshold for effective lubrication.
- The agitator connected to the worm gears output shaft experienced occasional high torque fluctuations, causing significant shaft deflection and radial runout. This disturbed the alignment of the mechanical seal faces, leading to intermittent contact and leakage.
- Installation and alignment of the mechanical seal required a dial indicator with a fixture to ensure the stationary face runout was within 0.05 mm. Given the large diameter (155 mm) and the confined space, achieving this precision was impractical under field conditions.
Successful Switch to Rubber Oil Seal with Skeleton for Worm Gears
Given the limitations of both packing and mechanical seals, I redesigned the seal arrangement using a skeleton-type rubber oil seal (lip seal) for the worm gears. Since the standard series did not include a 155 mm inner diameter, I specified a custom L-shaped rubber oil seal with a metal skeleton and a garter spring. The design was simple and cost-effective.
The performance of this seal type on the M20 worm gears reducer proved excellent. The reasons are as follows:
- The lip-type rubber oil seal has inherent self-tightening capability due to the spring-loaded lip, which maintains radial contact force even with slight shaft wear or misalignment.
- The seal is insensitive to moderate shaft runout and deflection—typical for worm gears in industrial agitators—because the flexible lip can follow shaft movements without losing contact.
- The low rotational speed of the worm gears (6 r/min) resulted in a very low sliding velocity at the sealing interface:
$$
v = \frac{\pi d n}{60} = \frac{\pi \times 0.155 \times 6}{60} \approx 0.0487\ \text{m/s}
$$
At such low speeds, wear is minimal, and the rubber lip can operate for extended periods without degradation.
- Installation was straightforward: the modified packing gland accommodated the oil seal directly, and no precision alignment was required.
Comparison of Seal Types for Worm Gears
To summarize the key differences among the three sealing methods applied to the M20 worm gears reducer, I present the following table:
| Seal Type | Leakage Control | Suitability for Low Speed | Resistance to Shaft Runout | Installation Complexity | Maintenance Frequency | Cost (Custom 155mm) |
|---|---|---|---|---|---|---|
| Packing seal | Poor after ageing | Moderate | Low | Easy (but cramped) | High (every few weeks) | Low |
| Mechanical seal (custom) | Unreliable at low speed | Poor (needs >0.5 m/s) | Very low | Very high (0.05 mm runout) | Moderate | High |
| Rubber oil seal (skeleton) | Excellent | Excellent | High | Low | Very low (long life) | Moderate |
From the table, it is clear that the skeleton-type rubber oil seal offers the best overall performance for the worm gears application, especially considering the low rotational speed and the non-standard shaft diameter.
Lessons Learned for Future Worm Gears Design
Based on this experience, I recommend the following for designers working with worm gears reducers:
- Standardize shaft diameters whenever possible. Using standard shaft sizes (e.g., 150 mm or 160 mm) allows the use of off-the-shelf seals, simplifying replacement and reducing costs.
- Select seal type based on operating conditions. For low-speed worm gears (below 0.5 m/s sliding velocity), lip seals (skeleton rubber oil seals) are far more reliable than mechanical seals, which require higher speeds to maintain a fluid film.
- Consider ease of maintenance. The limited space around the output shaft of many worm gears reducers favors compact seals that can be replaced without disassembling the entire coupling. The lip seal meets this requirement perfectly.
- Evaluate total lifecycle cost. Although a custom rubber oil seal may have slightly higher initial cost than packing, its dramatically longer service life and lower labor for replacement make it the most economical choice in the long run.
Conclusion
In summary, the M20 worm gears reducer’s packing seal failure was successfully resolved by switching to a skeleton-type rubber oil seal. This change eliminated oil leakage, reduced maintenance time, and improved equipment reliability. The key was matching the seal type to the actual operating parameters of the worm gears—low speed, moderate shaft runout, and confined installation space. The same approach can be applied to other non-standard worm gears reducers in similar industrial environments.