Actuators are on Track for theElectrified Public Transit Boom
Transit system designers have increasing needs for longer life and higher durability on components, particularly the actuators that enable safe, flex- ible connections to power sources. Fortunately,a new line of electric linear actuators is now available tomeet this need.
THE ROLE OF ACTUATORS IN
Actuators enable electric trams, buses, and trains toconnect and disconnect with power sources intermittently. This frequent connection and disconnection is not asnecessary in most conventional applications because theyrely on a single power source to which they connect oncein the morning and stay connected until the end of theirday. Hybrid vehicles, on the other hand, use battery powerfor routes that extend beyond city limits but switch back tocleaner and more economical overhead power many timesas they run their routes. This approach to charging the battery will become increasingly important as the number ofelectric trams, buses, and trains increases.
Actuators manage the overhead switching by controllingspring-loaded pantographs—mechanical assemblies thatsit on top of a vehicle and elevate above to connect withoverhead power lines. A spring raises the pantograph toconnect and maintain contact with the overhead powerline, and the actuator pulls the pantograph away from thepower source when switching to battery power. In otherdesigns, the actuator may pre-load the spring that raisesthe pantograph to the overhead power line. More efficientintermittent switching reduces the need to add aesthetically challenging and potentially dangerous overhead wiringwithin city limits as mass transit use increases.
Some designs use pneumatic actuators to manage pantograph connectivity, but the related air compression givesoff moisture that can cause clogging and, in low temperatures, icing. Pneumatic solutions also need many morecomponents, including pumps, pipes, and compressed airsystems. The increase in ridership compounds such issues,making pneumatic solutions increasingly less attractive astraffic increases.
Actuators also guide the switching to recharging stations, helping hybrid buses get the power they need to operate during the day. And for trains that get power fromthird rails, actuators control the extension of the contactsfrom the train to the powered rail.
Where existing applications might be demanding actuators with lifetimes between 20,000 and 30,000 cycles,daily cycle increases resulting from increased ridershipare expected to require actuators with lifetimes in the500,000 to 1,000,000 cycle range.
RISING TO THE OCCASION
Surviving such a high volume of cycles requires changesto the ball screw kit, motor design, braking strategy, andenvironmental resistance.
UPGRADED BALL SCREW KIT
The actuator ball screw kit includes the ball screw, nut,and bearings. Achieving up to a million cycles of life requiresmodification of the kit. Thomson Industries, Inc., for example, has been able to increase cycle capacity more than tenfoldby making three major design changes. The first change is using a larger diameter ball screw, strengthening the system atits core. The second change is implementing a multi-start ballreturn on the nut, which enables twice the number of ballsthat form the connection between nut and screw, and spreadsthe load more broadly across the balls. And, deployment ofangular bearings to the screw now provides greater stabilityand component stress reduction.
The motor design also impacts actuator life. Most actuators currently installed on electrified vehicles use traditionalcontact brush-based DC motors. But the brushes eventually wear out—much too soon for the extended use that thetransit designers are projecting. Replacing the brushes withan electromagnetic field is a much better alternative. Without friction from contact brushes, the motor can last virtuallyforever. There will, of course, be some wear on bearings orother parts of the motor, but the system longevity no longerdepends on brush life.
Using a brushless motor also contributes to a longer lifeby enabling electromagnetic control. In a pantograph, theactuator works against a spring. In one direction, it is the opposing force; in the other direction, it is an assisting force.An opposing spring provides stable, more controlled speedin one direction. An assisting spring, on the other hand, allows a more free-wheeling operation that must be controlled.Typically, the speed is controlled with a friction brake whenthe spring is assisting. But with extended use, friction brakeswear down and limit the life of the actuator. A better brakingapproach is to control the speed with the brushless motor anduse an electromechanical brake to hold the load in place oncethe actuator stops.
Actuators used in public transit applications are alreadysubject to government health and safety regulations, and expanded use will require maximum adherence. Where vehiclesin less-frequent operation might be able to use a lower ingressprotection level such as IP65 on current solutions, expandedoperation of a train moving at more than 60 miles per hourwould result in greater exposure to environmental forces andwould likely require a higher protection level. Such factors aredriving actuator suppliers to ensure that their materials andsealing strategies will protect for the 20 or 30 years that the vehicles themselves may last.
35 PERCENT DUTY CYCLE
When implemented together, the enhanced ball screw kit,brushless motor, electromagnetic braking, and environmentalresistance can increase the duty cycle considerably. High dutycycles enable more work to get done in a given timeframe without having to resort to forced cooling or using an actuator thatis stronger than the load requires, just to avoid overheating.
WHAT’S COMING DOWN THE ROAD?
In today’s transportation applications, actuators primarily provide the switching function and will continue to do so.However, switching requires some onboard electronics, whichenables actuators to communicate with each other and withother devices, providing operators with information such asfeedback on the position of the pantograph.
Although actuators used for charging stations are among thefirst components that must be upgraded to accommodate increased ridership and expanded electrification, other suitableactuator applications may soon follow, including door operation, step leveling, rail car connecting, and gap control for safeand easy passenger access. Until now, these applications haveprimarily been managed by pneumatic or hydraulic actuators,but with increased cycles and the requirement for more efficient and robust components, engineers should consider themore aptly suited electric linear actuators for their designs.
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