Orkot® Trunnion Bearing Friction Analysis

Most of the hydroelectric power plants currently operating in the United States are over 50 years old. They need to be modernized, refurbished, and upgraded to continue to work effectively. 

Bearings may be small pieces of a hydroelectric plant, but they’re crucial. Smooth operations rely on their ability to withstand immense force to avoid major consequences. A malfunctioning trunnion bearing can take down an entire radial gate. The ramifications of such a malfunction could include power loss, flooding, and reservoir lowering, not to mention the impact on downstream boat traffic and other recreation activities. 

Orkot® radial gate bearings are superior to bronze bearings because they are self-lubricating and run in dry conditions, in addition to other benefits. These features give them a lower coefficient of friction and make them longer-lasting. What does that look like in practice? This article presents data from a friction test of Lundevann Dam’s radial gate arms and shows precisely how much better Orkot® radial gate bearings perform than traditional greased bronze bearings.

Why Greased Bronze Bearings Fail

Greased bronze bearings were the standard when most hydroelectric dams were built decades ago, but they have several disadvantages. 

An issue with greased bronze bearings is that they are static. The lubrication on the bearings distributes itself to the low-pressure side, leaving the high-pressure side unlubricated. Grease escapes from bronze bearings, and when it does, corrosion occurs. 

Bronze bearings on a turbine wicket gate assembly at Guri Dam in Venezuela experienced failure due to corrosion. The thin piece of bronze was all that was remaining.

Radial gates are designed to resist water pressure, but friction forces from lack of lubrication and deterioration often aren’t taken into consideration. When they are, nominal friction coefficient values—around µ = 0.2 – 0.3—are typically used. But realistically, the friction value is much higher, and the greased bronze bearings aren’t equipped to handle it. The overload of friction most likely leads to breakage, and at least five hydroelectric plant failures have occurred due to deterioration from strain on the bearings.

Methods to Measure Radial Gate Arm Friction

To measure friction, research teams first install strain gauges to gate arms. These gauges measure the strain caused by bearing friction during gate movement. Then, the strain measurements from those gauges, along with the gate position, are digitally logged. The digital logging of data allows for precise measurement and minimizes error. Since this method makes it possible to measure strain while the gate arms are in service, it also avoids dewatering the gate or requiring stop logs—both of which are time- and cost-intensive. 

This method has proven to be effective and has been used to measure friction on 30 different bearings on 15 gates from 6 dams, including the trunnion bearing test at Lundevann Dam. 

The Lundevann Dam Trunnion Bearing Test

The Lundevann Dam powers the Åna-Sira power station in Norway and has two spillways, each with a surface radial gate. 

A research team measured radial gate arm friction under three different bearing conditions using:

1. The original bearings without prior lubrication 
2. The original bearings after lubrication
3. New self-lubricating Orkot® TLMM radial gate bearings

Friction measurements were taken over three years. The original bearings were measured in 1998. The measurements were then repeated in 1999 after the bearings were lubricated. Finally, the third set of measurements was taken in 2000 after the bearings had been replaced with new self-lubricating Orkot® hydro bearings.

Trunnion Bearing Test Results

The results found that the friction coefficient was significantly lower for the Orkot® Radial Gate Bearings. 

The graph below charts the friction coefficient for gate No. 1 at the Lundevann Dam. The friction on the original bearings in red was especially high, hovering around a constant of µ = 0.5. After thorough lubrication, the friction coefficient dropped by about 25% to between 0.45 and 0.35. 

The performance of the Orkot® radial gate bearings was unparalleled, with a near-constant friction coefficient of 0.13 and reaching as low as 0.09 on the left bearing.

The shape of the graph is important to note as well. An uneven shape indicates roughness of the bearing and uneven friction throughout the movement of the gate. In contrast, an even graph like Orkot®’s indicates a smooth bearing surface. 

Orkot® Outperforms Greased Bronze Bearings

Orkot® radial gate bearings greatly surpass the performance of the traditional bronze bearings based on this study of radial gate arms at Lundevann Dam. The grade of Orkot® material tested at Lundevann was TLMM, which the TXMM grade has an even lower level of friction and is what we now typically recommend in hydropower environments. 

These results are consistent with studies from other hydropower stations, like testing guide vane bearings at Dinorwig Power Station, which also found that Orkot® bearings performed better than greased bronze bearings.

Orkot® bearings are a proven way to ensure that hydroelectric dams continue functioning properly well into the future. By replacing bronze bearings with Orkot®, you could see massive savings on maintenance and upkeep costs along with additional cost-effective benefits.

Book a meeting with our Orkot® expert, Tom Carew, for a quote to upgrade your hydropower plant.