Gear drives and belt drives
In the transmission system of air compressors , there are generally two types: direct drive and belt drive. The superiority of one over the other has long been a point of contention in the industry. Direct drive in screw air compressors refers to the motor shaft driving the rotor via a coupling and gearbox, but this is not true direct drive. True direct drive means the motor and rotor are directly connected (coaxial) and both move at the same speed. This is obviously extremely rare. Therefore, the view that direct drive has no energy loss is incorrect.
Another type of transmission is belt drive, which allows the rotor speed to be changed by using pulleys of different diameters. The belt drive systems discussed below refer to state-of-the-art automated systems that meet the following conditions:
1. The belt tension in each operating state reaches the optimized value.
2. By avoiding excessive starting tension, the service life of the belt is greatly extended, while the load on the motor and rotor bearings is reduced.
3. Always ensure the correct pulley connection;
4. Replacing the belt is quite easy and quick, and no adjustments to the original settings are required;
5. The entire belt drive system operates safely and without faults.
It is worth mentioning that some manufacturers who advocate direct gear drives also use belt drives in some of their products.
Comparison of gear drives and belt drives
1. Efficiency
Excellent gear drives can achieve efficiencies of 98% to 99%, while well-designed belt drives can also reach 99% efficiency under normal operating conditions (see Professor Heinz Peeken's research report published in the December 1991 issue of "Transmission Technology"). The difference between the two does not depend on the choice of transmission method, but rather on the manufacturer's design and manufacturing capabilities.
2. Idle energy consumption
For direct gear drives, the no-load pressure generally needs to be maintained above 2.5 bar, and sometimes even as high as 4 bar, to ensure gearbox lubrication.
For belt drives , theoretically the no-load pressure can be zero, because the oil drawn into the rotor is sufficient to lubricate the rotor and bearings. However, for safety reasons, the pressure is generally maintained at around 0.5 bar.
Taking a 160kW gear-driven air compressor as an example, if it works for 8,000 hours a year, with 15% (1,200 hours) of that time being idle, this machine will consume 28,800 kWh more electricity per year than a belt-driven air compressor of the same power (assuming the no-load pressure difference between the two machines is 2 bar, approximately a 15% difference in energy consumption). In the long run, this will be a considerable expense.
3. Oil loss
Experienced users know that in the event of a loss of lubrication, the gearbox will be the first to suffer. Belt-driven systems do not have this safety issue at all.
4. Design working pressure according to user requirements.
The operating pressure required by users is often not exactly the same as the pressure of the manufacturer's standard model. For example, if a user requires a pressure of 10 bar, depending on the condition of the aftertreatment equipment, the length of the piping, and the degree of sealing, the required operating pressure of the air compressor may be 11 or 11.5 bar. In this case, a 13 bar rated air compressor is usually installed, and the outlet pressure is set to the required operating pressure on site. In this case, the discharge volume will remain essentially unchanged because although the final operating pressure is reduced, the rotor speed does not increase.
Modern belt drive designers can easily modify the pulley diameter and design the operating pressure to perfectly match user requirements, allowing users to obtain more airflow with the same motor power. Wheel drives, however, are not so convenient.
5. Pressure change of an already installed air compressor
Sometimes, due to changes in the user's production process conditions, the original design pressure of the purchased air compressor may be too high or too low, and the user may wish to change it. However, for gear-driven air compressors, this task can be very difficult and expensive, while for belt-driven air compressors, it is a piece of cake, requiring only the replacement of the pulley.
6. Install new bearings
When rotor bearings need replacement, geared air compressors require a major overhaul of both the gearbox and its main shaft bearings, a cost that is unacceptable to users. This issue does not exist for belt-driven air compressors.
7. Replace shaft seal
All screw air compressors use an annular shaft seal, which needs to be replaced after a certain lifespan. For gear-driven air compressors, the motor and coupling must be separated before accessing the shaft seal, making this a time-consuming and labor-intensive task, thus increasing maintenance costs. For belt-driven air compressors, it is much easier to simply remove the pulley.
8. Damaged motor or rotor bearings
In gear-driven air compressors, damage to the motor or rotor bearings often affects connected critical components, causing both direct and indirect damage. This situation does not occur in belt-driven air compressors.
9. Structural noise
For gear-driven air compressors, since the motor and rotor are rigidly connected, the vibration of the rotor in the compression chamber is transmitted to the gearbox and motor bearings. This not only increases the wear of the motor bearings, but also increases the machine noise.
