For many years Nexen has been promoting the benefits of these air-engaged clutches and brakes. Yet some people in the industry still perceive electrically-actuated brakes as more effective, based on the fact that electricity is simple and convenient to operate. We agree electrically-actuated clutches/brakes are simple, but compared to air-actuated clutches/brakes, they don’t measure up in terms of efficiency or cost-effectiveness.
In fact, when you look at life performance, air-engaged clutches/brakes, come out way ahead. They typically have twice the life cycle of electrical clutches/brakes.
This article is based on actual performance results of a test conducted by an independent testing lab, between air-actuated and electrically actuated clutch-brakes. It gives comparative data in the following areas:
- Response Time
- Clutch and Brake Overlap
- Torque Output and Fade
- Friction Facing Life
- Electrical Energy Consumption
- Overall Power Consumption
The results clearly show that air-actuated clutch-brakes outperform electric clutch-brakes, and offer superior performance. In fact, Air Champ® clutch-brakes typically have twice the friction facing life cycle of electrical.
The Air Advantage
Pneumatic, or air, clutch-brakes are more efficient and economical than similarly sized electrical clutch-brakes because of one important variable — air’s responsiveness to heat.
Air does not generate heat during clutch and brake engagement. Less heat allows greater torque transmission and efficiency. The greater thermal capacity of air-engaged clutch-brakes gives them longer operating life. Since you can “get more work” out of an air-actuated clutch-brake, air proves to be more cost-effective. It also costs less to run an air compressor than to operate an electrical unit.
Nexen Air Champ® products have simple designs which demand fewer parts than electrical units. This represents a significant cost savings (up to 80% over electric clutch -brakes) in the long run.
The following test was conducted under controlled test tab conditions. The two products tested were Nexen’s FMCBE-625 and a popular electrically-actuated clutch-brake. The first area we tested was response time.
Specifically, response time is the increment of time in seconds from the time power is turned on or off at the control valve, or power supply, to the time the clutch or brake responds with (full load) torque, or disengages and the torque begins to decay.
For air-engaged units, the size of the control valve is a contributing factor to the response time value. A valve with a higher flow of capability, or Cv factor, is desired for faster response time.
The derivation of response time data for a specific clutch-brake and control valve is typically measured under controlled test-lab conditions. The Nexen unit was controlled with a 4-way valve and with quick exhaust valves installed in the air inlets.
The electric unit was connected to a standard 90 volt power supply.
|Model||Clutch Response Time||Brake Response Time|
|Nexen FMCBE-625||0.072 Seconds||0.056 Seconds|
|Comparable electric clutch/brake||0.102 Seconds||0.080 Seconds|
Obviously, the air-engaged unit was faster. This was partly due to the air-engaged unit’s ability to bring the load up to speed with less slip at the interface.
Clutch and Brake Overlap
Obviously, the air-engaged unit was faster. This was partly due to the air-engaged unit’s ability to bring the load up to speed with less slip at the interface.
In air-engaged units having separate cylinders for the clutch and the brake, the tendency of overlap occurs because full supply pressure forces air into one cylinder at the same time the pressure in the other cylinder is pushing the air to exhaust. In air-engaged units, this condition is controlled with quick exhaust valves located in the air inlet port.
Electric clutch-brakes apply coil suppression techniques to control overlap. Coil suppression is defined as the use of a diode to direct the flow of reverse to ground. Without suppression in the control circuit, an arc results from the strength of this current flow, resulting in simultaneous clutch-brake engagement. Even with coil suppression, a small amount of overlap can occur.
The difference between Nexen’s FMCBE and the electric clutch-brake is that Nexen is designed with only one piston which totally eliminates the possibility of overlap.
With Nexen’s FMCBE it is physically impossible to have a simultaneous clutch-brake engagement because the piston shifts one direction to engage the clutch and the other direction to engage the brake.
Torque Output and Fade
Next, we compared the torque output of eight-inch diameter interface between air and electric clutches. Figure 4a shows that an electric clutch which ran 1,000 rpm, or 2,000 fpm, transmitted only 60% of its rated torque.
In comparison, the pneumatic clutch transmitted 35 to 40% more torque under the same conditions because it operated at a lower temperature. Note the dramatic drop in the electric unit torque when the speed increased.
The advantage of this kind of performance characteristic in the air-engaged clutch is that smaller air units can be used in place of larger electric units.
Also note the difference between the rated static torque and measured static torque in Nexen’s and electric clutch-brakes.
As with torque output, the torque fade differs substantially between air and electrically-actuated clutch-brake performance.
All friction clutches and brakes slip during engagement. Some of the kinetic energy produced during slip converts to heat at the dynamic interface. This heat has a negative effect on clutch-brake performance because the friction lining’s coefficient of friction goes down when the temperature elevates, resulting in a torque decrease. This is known as “torque fade.”
Excess heat greatly reduces the performance of clutches and brakes. The more heat that is introduced, either by friction or an electrical coil, the more limited the clutch or brake’s ability to deliver torque. Electrically-actuated clutches are engaged by continuously passing an electrical current through an electromagnetic coil. This current elevates the temperature of the entire unit even before any load is applied.
However, air-engaged brakes are engaged with static, pressurized air contained in a cylinder. Static air maintains a constant force. Consequently, little energy is consumed.
Facing Life Comparison
Next we looked at the facing life differences between FMCBE-625 and the electric clutch-brake.
To determine the estimated friction facing life we followed this procedure:
- First, find the volume of the useable facing material.
- Second, determine the friction material wear rate.
- Third, determine the work energy capacity. This capacity is the quantity or amount of energy that can be absorbed by the friction lining before it becomes consumed or worn away.
- And last, calculate the energy per cycle for a specific application. This is the amount of energy generated each time the clutch or brake is engaged. It is an estimate based on the size of the inertia load and the speed at which the load is applied.
Based on example of 38.56 ft. lbs. of energy per cycle at 60 cycles per minute for an eight hour day the Nexen unit has considerably more available facing volume, which translates into more horsepower-hours. The life in days is based on 60 cycles per minute, 60 minutes per hour for an eight-hour day. The comparative charts on the next page (Figures 10a and 10b) show how we applied the mathematical steps explained earlier to both the Nexen FMCBE-625 and the electric clutch-brake.
The energy absorbed per cycle by the clutch-brakes when applied to the conveyor system was 38.56 ft. Lbs. Dividing the work energy capacity of the friction material by the energy per cycle gave us the maximum number of cycles that each clutch-brake is capable of before replacement is necessary.
|Model||Facing Volume||Horsepower-hours||Life (days)|
|Nexen FMCBe-625||0.712 inch³||203.0||362|
|Comparable electric clutch/brake||0.111 inch³||31.7||56|
Electrical Energy Consumed
Using the friction facing comparisons we determined the amount of electrical energy required to operate an air-engaged unit vs. an electrically-engaged unit.
First, we compared the amount of electrical energy consumed by a compressor used to produce the 30 psi required of the clutch-brake with the electrical energy consumed by the electric unit’s coil.
Next, we looked at the air-engaged unit to determine the amount of free air consumed per cycle and the horsepower to compress this free air. We converted compressed air to free (uncompressed) air based on 14.7 psi of atmospheric pressure (or absolute pressure).
To arrive at the actual power consumption of the air-engaged clutch-brake, we multiplied the horsepower that operates the compressor by 745.7. The result was 6.38 watts.
In contrast, the electric clutch-brake power consumption, published in the manufacturer’s catalog was 18 watts.
Since electrical energy is purchased in kilowatt-hours, we converted watts to kilowatts or kilowatt-hours by dividing the watts by 1,000.
OUR FINAL RESULTS: IN THE CONVEYOR APPLICATION, THE ELECTRICAL UNIT COIL CONSUMED 2.5 TIMES MORE ENERGY THAN THE COMPRESSOR THAT PROVIDED AIR TO THE CLUTCH-BRAKE .
Why Air is Better
Based on the entire test analysis results, the benefits of air-actuated clutch-brakes over electrically-actuated clutch-brakes are:
- Up to 30 percent faster response time.
- Up to 40 percent more dynamic torque.
- Up to 6 times longer friction facing life.
- Up to 2.5 times less energy (electrical) consumption.
- Up to 30 percent more thermal horsepower.
No other clutch-brake manufacturer can offer you innovation like Nexen. With the Nexen Air Champ line, you can improve your business through efficient production and efficient costs. It just works better for you.
But product efficiency is just one example of Nexen’s leadership in the industry. We have an ongoing commitment to use the latest research, technology and manufacturing methods to produce products that offer the best performance and applications in the world.
We intend to stay ahead of the game. As we do, we will always keep you educated and informed of our innovations designed with your production needs in mind.
ISC Offers Complete Design Services
ISC Companies’ Machining & Fabrication Group is proud to be able to offer our customers design and documentation services using SolidWorks™ and AutoCAD™ software. With trained designers on staff at our Minneapolis Headquarters we are expanding beyond creating documentation for in house design projects and offering design/documentation as a standalone service to our customers.
Our capabilities include:
- 3D modeling
- Digital prototyping
We began using AutoCAD™ then expanded to SolidWorks™ to create needed documentation for use in our own machine shop for shop drawings; computer aided machining (CAM) and for documentation of customer supplied materials. As our capacity in this area has increased we have been able to offer design services to an increasing number of customers.
Our design service takes many forms. Some customers look to ISC to create prints for machine parts that are no longer supported with spare parts from the OEM. This usually means doing some detective work with a broken or worn-out (or both) part and recreating what is needed from both a dimensional and functional standpoint. But there’s more. A lot of machining and welding technology has changed in the last few years. By applying our expertise in these areas we can often make recommendations to improve these parts or shorten their manufacturing time saving hundreds if not thousands of dollars.
For other customers we have been the collaborative agent who takes their ideas, often in the form of “napkin sketches” and brings their design ideas to life. And with the depth of experience we have in product applications we are usually able to offer some ideas and perspectives that may well have been otherwise missed.
For more information about how we can help in your design process contact your ISC Companies representative today.
The following is a white paper from Omron STI
Type 2 versus Type 4 Light Curtains
As machinery safety standards and safety light curtains evolve to meet new application demands, users are faced with more choices and responsibilities than ever before.
A new breed of light curtain is gaining recognition in the United States. Developed in Europe and classified as “Type 2,” it is a lower-cost, reduced-capability alternative to the more robust “Type 4″ high safety performance level light curtains typically used to safeguard machinery in the United States. The terms and definitions of the product “Type” are derived from the international standard for light curtains, IEC 61496. Understanding the capabilities and differences between these two types of machine safeguarding devices will help users determine which is right for their application.
In most instances, Type 2 and Type 4 safety light curtains look much the same.
However, these photoelectric safeguards are designed to satisfy vastly different safety requirements. Essentially, Type 2 products are designed to a lower level of safety integrity and must not be used in applications where a Type 4 control is the appropriate choice. Although the differences are technical and based on various industry standards, these devices differ in three significant areas:
1. Fault Detection Circuits
Type 2 light curtains lack the redundant automatic self-checking circuits employed in Type 4 light curtains. As a result, the Type 2 light curtain does not meet the OSHA or ANSI standard for the highest safety performance level. Type 4 safety light curtains are designed to immediately detect the failure of a single component within a defined response time. This is not true of Type 2 light curtains.
2. Optical Angle
Traditional Type 4 safety light curtains have an effective optical angle of ±2.5 degrees, while Type 2 devices have an effective optical angle of ±5 degrees. The wider optical angle increases the possibility of reflective surface interference, where a reflective object near the sensing field of the light curtain causes an optical “short circuit.” As a result, an object in the sensing field may not be detected, as the light “bends” or reflects around the object.
This possibility demands users take great care during installation and alignment to ensure proper operation of the Type 2 device. Fortunately, there are simple tests to detect this potential hazard. The tests must be performed during installation and periodically afterwards for any light curtain.
The third difference is price. Type 2 devices are typically 15% to 30% less expensive when compared to an equivalent Type 4 device. The cost difference stems from the less precise optical angle and fewer fault detection circuits. In addition, Type 2 light curtains typically have fewer available features, such as exact channel select, floating blanking, MPCE (Machine Primary Control Element) monitoring and MTS (Machine Test Signal).
Application examples – type 2 or type 4, when to use
Determining when to use a Type 2 or Type 4 safety light curtain may best be demonstrated by reviewing a couple of examples. While it is essential to perform a complete risk assessment on all machines, the severity of the potential injury is the overriding factor when deciding between a Type 2 and Type 4 safety light curtain.
First, a pharmaceutical company’s packaging department uses index tables, conveyors, filling and labeling equipment and a multitude of moving parts.
In the assessment process, the user determined that the size and force of the motors used on the index table and conveyor was insufficient to cause serious injury. The worst-case injury was defined as a potential bruise requiring simple first aid.
In this application, a Type 2 light curtain is the light curtain of choice. It serves as an appropriate safeguarding device, while doubling as a process control device.
In our second example, the assembly department of a gas and pneumatic regulator manufacturer requires that an operator continuously interface with the assembly equipment.
LThe workstation consists of a small pneumatic press and an automatic self-feeding screwdriver. The operator must insert a multitude of parts that need to be compressed while the driver inserts screws.
The worst-case injury would require off-site medical attention.
In this application a Type 2 device is not an appropriate safeguarding choice, but rather a Type 4 light curtain is recommended.
The Occupational Health & Safety Act (OSHA) and the American National Standards Institute (ANSI) both require the highest level of safety performance for safety-related machine control systems when serious injuries can occur.
Examples of machines that require the highest safety performance level include machine tools, such as power presses, shears, press brakes, robots, etc.
A Type 4 safety light curtain employs self-checking circuitry to monitor itself for internal faults. If it detects an internal fault, the Type 4 safety light curtain immediately sends a stop signal to the guarded machine and the light curtain enters a lockout condition. Only after replacement of the failed component, and an appropriate system reset, will the Type 4 light curtain and the guarded machine be restored to operating condition.
Because Type 2 light curtains lack the redundancy of internal fault detection circuits, they cannot achieve a sufficiently high safety performance level and therefore are not suitable as a safeguarding option on machinery where OSHA or ANSI requirements or risk assessments require control reliability. Remember also that a Type 2 light curtain is not protected against dangerous failures when exposed to extreme levels of electrical interference sometimes found in industrial environments.
Don’t roll the Dice – Perform a risk Assessment
Conducting a thorough risk assessment requires the user follow a formal procedure that considers many factors when looking at machinery hazards. A risk assessment must be applied in a consistent manner across all plant machinery. This will enable the user to logically evaluate safety hazards and hazard-guarding solutions. The process considers all hazards and each type of safety hazard on a given machine.
The risk assessment analyzes each hazard and estimates the risk level by breaking it down into three components: Frequency of exposure, Probability of injury, and Severity of the potential injury.
An operator, for instance, typically has a high level of exposure, while someone performing maintenance does not. Probability considers machinery speed, and compares it to a person’s typical reaction time – so a fast-cycling machine will have a higher probability of injury than one that is a relatively slow. The user must also estimate the type of potential injury in terms of severity, ranging from a simple pinch on the low end, to loss of a digit or even life at the other extreme.
Severity of injury should always dictate the assessed risk level. If the severity of potential injury is high, but exposure and probability are low, a Type 2 device is not an appropriate safeguarding option. Type 2 devices are not intended for use where ANSI B11.19, OSHA 1910.212 or 217 apply, and should never be used on a mechanical power press. Type 2 devices are not and cannot be made Control Reliable.
A Question of Interpretation
Because of these differences, Type 2 light curtains are intended for use in machine-guarding applications where the worst-case injury resulting from an accident may be remedied by simple first aid.
The social, legal and political cultures of Europe and the United States are distinct, resulting in different interpretations of “simple first aid.” These differing interpretations impact a user’s decision as to whether to apply a Type 2 or Type 4 device in a given application.
In Europe, first aid is measured, in part, on the amount of time an employee misses from work. For example, if a worker is injured and must go to the hospital for stitches or other medical procedures, yet returns to work the same day or the next day, it would be considered simple first aid.
In the U.S. these injuries would be considered much more serious due to the nature of the injury itself, lost machine and worker productivity (the “gawk” factor, lower employee morale, investigating the cause of the injury, etc.), required injury reports, preparing insurance claims and so on.
In the U.S., first aid is defined in OSHA 1904.12. as any one-time treatment, and any follow-up visit for the purpose of observation of minor scratches, cuts, burns, splinters, and so forth, which do not ordinarily require medical care. Such one-time treatments and follow-up visits are considered first aid, even though they are provided by a physician or registered professional personnel.
Based on the risk assessment results and the type of hazard, the user can work with a safety expert to determine the most appropriate machinery safeguards for each application.
Remember that a light curtain, whether Type 2 or Type 4, may not be right for every machine safety application. Other safety equipment, such as safety mats, safety switches, hard guards or a combination of equipment may offer the optimum solution.
Machine users should reference ANSI B11.TR3, ANSI/RIA R15.06-1999, and/or ISO 14121-1 prior to beginning their formal risk assessment. A comprehensive discussion of safety strategy and risk assessment, including formal procedure documentation, is available at http://sti.com/safety/.