When comparing brakes (unwinds) and clutches (rewinds) to motors, there are significant advantages and disadvantages that must be considered to determine the best component for your application currently and in the future. Here we take a quick look comparing brakes and clutches to motors in 7 key categories.
Acceleration
Unlike a brake, a motor can drive a roll of material to maintain precise tension during the acceleration of the roll. This is very important if you are running large rolls of very delicate materials (such as tissue paper) or materials that are very extensible. A driven unwind can also help with achieving proper registration during acceleration faster than a brake system, although this usually requires the addition of a servo, which can be more complex to install, commission, and maintain. Advantage: Motors
Cost
Motors are significantly more expensive when compared to a brake or clutch. Not only are the individual components as much as 2-3 times more than a brake/clutch, the installation and service usually require personnel that are very familiar with drive controllers, PLCs, etc. When comparing brakes/clutches to motors/drives always look at the total system cost plus support, and how easily the system can be upgrade modified later on (if ever needed). Advantage: Brakes and Clutches
Maintenance
Drives and PLC’s are constantly changing. As maintenance and service personnel change in your facility, new personnel are often not familiar with the older brands and software version on the installed motor system. This will often require calling in service that are very familiar with the drive but may not be familiar with the tension control portion or your particular application. So motor/drive maintenance costs could add up quickly. However, motors themselves require zero to no maintenance. Brakes and clutches require very minimal maintenance and are very easy to maintain by any of your personnel. Advantage: Brakes and Clutches (depending on your control system)
Installation
Motors generally require a gearbox and a more involved installation compared to brakes and clutches which directly mount on to the end of your unwind or rewind shafts and bolt to your machine frame. Advantage: Brakes and Clutches
Torque Requirements
Motors operate within a very limited torque/speed range, whereas brakes and clutches feature a much wider available torque range. This benefit depends on the tension requirements of the material or materials you are currently processing, as well if you plan to run different materials on the same machine in the future. An example of the advantage of a wider torque can be seen in the following:
If running a 3” wide by 50” diameter roll of material and a 45” wide by 50” diameter roll on the same machine, where web speed is 1800 FPM, tension is 1 PLI, and outside diameter of core is 4”, you have the following scenario:
Minimum torque requirement:
- 3 (1 PLI x 3 inch width) x 2 (radius of the core) = 6 in-lbs.
Maximum torque requirement:
- 45 (1PLI x 45 inch width) x 25 (radius of the full roll) = 1125 in-lbs.
The overall ratio of torque that the application requires is:
- 1125/6 = 187:1
Since the average range capability of a drive motor is only about 25:1, it is impossible to accomplish using a motor, however is easily achievable with a brake or clutch, for example, Montalvo’s V Series Brakes. Advantage: Brakes and Clutches
Energy
When running at a constant speed and the motor is acting as a brake, it can dissipate energy as electricity as opposed to heat (brakes/clutches). However, this is only advantageous if you can economically utilize the generated electricity. Advantage: Motors (if can utilize electricity)
Controls
Motors can be used to either pull (rewind) or brake (unwind), however they require a more sophisticated control method, whereas Brakes and Clutches are compatible with a range of different control methods. Advantage: Brakes and Clutches