Test Drive the DBS Drive Unit
By James Hu, PE, Senior Project Engineer
Traditionally, the capacity and performance of a high-torque low-speed clarifier and thickener drive unit are calculated by the design engineers of the manufacturer. The results, in most cases, are good enough for typical applications. However, to optimize the drive unit design, and ensure its superior performance under the most severe service conditions, there is no better way than a dynamic load test. DBS has built a dynamometer just for that.
There are many reasons to perform a dynamic field torque test. The test can detect potential problems before the equipment is put into service. It is much easier to fix the problem sooner than later. The torque test is also used to prove that the drive is capable of producing the manufacturer’s claimed torque. The test demonstrates the structural integrity of the drive. It shows that the drive unit can be safely loaded to its rated torque. In addition, it demonstrates that the rake arms are capable of handling the required load. The accuracy of the DBS torque gauge is verified against the load cell readings. If the gauge is found to be off, it can easily be calibrated.
The DBS dynamic torque test machine measures 18 feet long, 6 feet wide and 8 feet tall. It is equipped with two long-stroke hydraulic cylinders, each is capable of 31,000 pounds of force. The body of the cylinders and the flange of the drive unit are fastened to the dynamometer frame; and the rods of the cylinders are connected to the rotating drum of the drive unit. As the drive unit turns, it pushes on the rods of the cylinders and expels the oil out of the cylinder chambers through an adjustable needle valve, which regulates the back-pressure on the cylinder pistons. The back-pressure is indicated on a pressure gauge and is used to calculate actual torque by multiplying the area of the pistons and the torque arm.
The test generates valuable information for engineers to understand how the drive unit performs under load and to verify the design against the experimental data. Through many tests, the engineers at DBS have collected a substantial amount of data, which is being compiled for design optimization. Most importantly, the integrity of the products is tested against the strictest design specifications.
Those who are familiar with field torque tests may want to know the difference between a dynamometer test and field torque test. The dynamic field test requires long-stroke cylinders similar to what are used on the dynamometer. They are heavy to transport, time consuming to set up, and expensive to perform. Therefore, most field tests are static, which only require a much lighter and simpler test rig. The static test is good for the rake and scrapers, but it tells little about the running performance of the drive. The dynamic field torque test is better, but structural deflections in the rake often confuse the results. It is also a little too late or complicated if a problem is found in the field test. The drive units that have already been calibrated on the dynamometer will not only assure success in the field test, but also allow the clarifier manufacturers to conduct field torque tests by simply anchoring the rakes to the clarifier floor and using the torque gauge as a load indicator.
The dynamometer has also paved the way for more extensive research: such as load sharing in multiple-pinion drive units and stresses in the main gears and pinions.
Performing the Test
At the heart of the dynamic torque test kit are two long stroke hydraulic cylinders. The cylinders are lightweight and compact, allowing them to be lowered into the tank manually without the need to rent expensive cranes. Set up is a breeze, just attach the cylinders to the tank floor with half inch concrete anchors and attach the other end of the cylinder to the rake arms with supplied nylon straps. Using nylon straps instead of chains ensures that the rake arm coating is not damaged. It takes just two people to perform the test. One operator controls the load on the rake arms while simultaneously observing the reading of the drive unit torque gauge. The operator controls the hydraulic backpressure, and therefore load, on the cylinders by adjusting a needle valve. Torque load on the drive unit is calculated by multiplying the load on the cylinder by the distance from the center of the tank to the point on the rake arm where the cylinder is attached. The other operator starts and stops the drive unit, making sure the cylinders do not bottom out. The entire test, from set up to break down, can be performed in a matter of hours. The test rig is routinely weight test calibrated to ensure maximum accuracy.
In October 2009, DBS replaced two Envirex model C40 units with D30-BE’s on Ø80 ft primary clarifiers at Hagerstwon, MD Wastewater Treatment Plant. The consulting engineer specified a dynamic field torque consisting of pulling a weighted sled with the rake arms. DBS proposed an alternative test which was accepted. The DBS engineer successfully performed the tests and demonstrated that the torque gauges were accurate within 1% of the gauge’s full scale reading.