Self-Adjustable Motor Base Article

Authored by Jesse Dupuis and Don Sullivan

Regal Beloit Corporation is a leading manufacturer of electric motors, electrical motion controls, power generation and power transmission components.

Topic: Are Your Commercial Motor Maintenance Assumptions Off Base? Your Motor Base Could Be Robbing You Of Performance and Efficiency?

Article Abstract - This article will explore the different types of commercial HVAC motor bases and the vast differences they have in maintainability and ultimate drive efficiency. A detailed analysis of a spring loaded motor base technology in comparison to industry standard (threaded bolt adjustment) bases will highlight the energy savings, performance improvement and payback of retrofitting with a spring loaded base.

In HVAC belt driven applications, belt tension represents a significant piece of the system’s overall efficiency. That efficiency can be adversely affected by belt slippage or excess tension. How that belt drive is installed and maintained is typically a function of the ease of use of the motor base.

Commercial HVAC Motor bases typically fall into two categories. The first category is known as a fixed-position base. These bases typically have one or two adjusting points where a nut is turned clockwise or counter clockwise to move the motor forward or backward. As the motor moves forward during adjustment the drive tension decreases as the sheaves (pulleys) move closer to each other. Conversely, as the motor moves backward the sheaves move further away from each other and tension on the belt increases. The U.S. Department of Energy publishes that notched v-belt drive efficiency “deteriorates by as much as 5% (to a nominal efficiency of 93%) over time if slippage occurs because the belt is not periodically re-tensioned.”

The second type of motor base is an automatic or tension controlled base. A popular design is a spring-loaded version that automatically adjusts for changes in center distance. When a motor operates, the starting torque causes the drive to flex slightly forward and once inertia is overcome it rebounds into position. Automatic tension-controlled bases with a spring design act as a constant resistant force that ensures the motor’s v-belt drive is constantly under adequate tension.

System Efficiency vs Proper Belt Tension

Under-tensioned belts result in slip between the driver sheave and the driven. The outcome of this is:
1. Decreased airflow, which can either increase runtime or reduce occupancy comfort.
2. Decreased power transmission efficiency and increased belt temperature due to the friction caused by the slippage.
3. Decreased overall component life.

Alternatively, over tensioned belts result in:
1. Decreased bearing life
2. Excessive heat generation
3. Increased motor energy consumption

Consider this a Goldilocks and the Three Bears analogy. The ideal belt tension is the point in which you have minimal slip and proper power transmission, producing proper airflow with the minimum energy required to do so. The result is the belts operate at the perfect nominal temperature. Anything more or less would either be too hot or too cold for Goldie.
Achieving and consistently maintaining this level of tension when using a fixed-position base can be a difficult task. It requires proper installation and periodic maintenance best practices to ensure that the belt drive maintains an adequate level of efficiency. Manufacturers’ recommendations for installing belts and re-tensioning are to “Check tension frequently during the first 24 hours of operation. Check after jog start or 1-3 minutes of operation, at 8 hours, 24 hours, 100 hours and periodically thereafter are recommended”. 

(Figure 1) A difficult task to be sure!

Figure 1 – Belt Tension vs Time for no re-tensioning and quarterly re-tensioning

The U.S. Department of Energy publishes that “The most important thing to control in a V-belt drive is the tension. If belts are too loose, they tend to vibrate, wear rapidly, and waste energy though slippage. If they are too tight, they will also show excessive wear and can dramatically shorten bearing life through exces¬sive lateral loading.” The loss in efficiency due to slipping v-belts can be calculated and used for Return on Investment (ROI) calculations.
In an ideal world, belt tension should be checked as often as possible to ensure there is no degradation of belt tension and thus no loss in efficiency or airflow (see figure 2). The fact that a belt drive is rubber riding on steel means it will experience material loss as the components rub over one another. Not to mention the belt length elongation potential with some belts manufactured with polyester cords. When one considers the costs of providing this level of attention to detail to ensuring top performance one typically comes to a compromise that falls somewhere in between poor performance and minimal labor cost.

Figure 2 – Airflow and efficiency loss due to lack of adequate belt tension

Consider this - if an HVAC contractor bills out at a rate of $75/hr and is required to re-tension belts per the manufacturers’ recommendations and every quarter thereafter, the cost to do so is as follows:
Typical time to re-tension belts = 15 min
Number of times required in a year = 1-3 minutes of operation, at 8 hours, 24 hours, 100 hours and quarterly thereafter = 7 times per year
Rate = $75 / hr
Total Annual Maintenance Cost = $75 x .25 x 7
                                                   = $131
Total Time Commitment = 1 ¾ hours

Note: The cost outlined above is per person. Oftentimes re-tensioning is done by 2 people, but for the purposes of this analysis, one will just be considered.

Bearing in mind the amount of belt driven equipment in a typical commercial facility, which can reach the hundreds, the time to re-tension belts can be daunting. As such, quite a few facilities don’t participate in best practices and some don’t re-tension at all (see Figure 1).

The motor base used in HVAC applications can either add to or take away from the problem. Typical fixed position motor bases which are used in almost all of these situations have their own issues relating to a contractor’s ability to maintain proper tension. They are notoriously difficult to align the belts properly, they often seize and are impossible to move or at best are difficult to adjust. This all makes the process for proper re-tensioning all but impossible.
As mentioned previously, The U.S. Department of Energy states that notched v-belt drive efficiency “deteriorates by as much as 5% over time”. They also publish that a 100HP motor operating 24/7 that operates at a load of 75% will consume approximately 527,000 kWh per year. In this example, a drive that has recovered lost belt tension of 5% would equate to a reduction of energy use totaling about 26,888 kWh annually and producing a savings of roughly $2,600 at just $0.10 kWh. If the fixed position base is not routinely maintained, the potential ROI for upgrading from a fixed position base to an automatic tension-controlled base would typically be under one year.

Some Original Equipment Manufacturers (OEMs) design their HVAC equipment such as blowers, to include an integrated motor base. These bases can vary in both fixed position and self-adjustable designs. Retrofitting or replacing the OEM bases can often be completed with aftermarket off-the-shelf bases, but may require minor removal of spot welds and drilling of holes to install.

With a spotlight now being placed on HVAC system efficiency, motor bases which are out-of-site out-of-mind, heavily impact the overall performance of belt driven HVAC equipment. Upgrading to self-adjustable motor bases requires mechanical installation know-how which is typically possessed and easily accomplished by existing maintenance staff or HVAC authorized contractors. They virtually eliminate the time and cost of maintaining belt driven HVAC equipment all while ensuring optimal functionality and proper airflow are continually provided. The payback savings and increases in performance rival that of other sexier high performance technology currently spreading like wildfire in the industry.

  • U.S. Department of Energy – Motor Tip Sheet #3 DOE/GO-102000-0972
  • Browning® Belt Tension – Form 5453
  • U.S. Department of Energy – Premium Efficiency Motor Selection And Application Guide DOE/GO-102014-4107
  • U.S. Department of Energy – Energy Tips Motor Systems DOE/GO-102005-2060
About Regal Beloit Corporation
Regal Beloit Corporation (NYSE: RBC) is a leading manufacturer of electric motors, electrical motion controls, power generation and power transmission products serving markets throughout the world. The company is comprised of three business segments: Commercial and Industrial Systems, Climate Solutions and Power Transmission Solutions. Regal is headquartered in Beloit, Wisconsin, and has manufacturing, sales and service facilities throughout the United States, Canada, Latin America, Europe and Asia. For more information, visit


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