Removing ice from airplanes before takeoff is a crucial safety precaution that requires dependable equipment. Otherwise, equipment failures can delay hundreds of passengers and divert ground crews from their primary work.
Simon Aviation Ground Equipment Inc., Olathe, Kans., equips its deicer trucks with advanced systems for heating and delivering the deicing fluid. From a telescoping boom, an operator sprays hot deicing fluid or ambient temperature anti-icing fluid on the airplane, Figure 1. But coupling failures in the truck’s hydraulic system caused a decline in its service availability.
The 275-hp truck engine propels the vehicle and powers all hydraulic functions through two systems: drive, and pumping and heating. Through power take-off (PTO), the drive system governs truck speed at 4 mph during deicing; and actuates aerial boom and basket movements.
The pumping and heating system, which delivers the fluids to spray nozzles in the basket, is driven by a hydraulic pump directly coupled to the truck engine. The pump operates four hydraulic motors, which drive the heater blowers and the pumps for Type I (deicing) and Type II (anti-icing) fluids.
In the deicing system (where the failures occurred), a hydraulic axial piston motor powers a gearbox-driven centrifugal pump. The system pumps 110 gpm of deicing fluid to a 5 million Btu/hr heater. This unit heats the fluid so ground crews can spray it at 180 F, melting ice from the aircraft.
After two years, Simon noted a decline in the service availability of its trucks. Engineering manager Rick Smith explains, “The Type I system had a weak link,” namely an elastomeric shear-type tire coupling connecting the hydraulic piston motor and centrifugal pump. Problems ranged from relatively harmless failure of the rubber element to violent destruction of the entire coupling.
According to Mr. Smith, torsional vibration generated by the 3,100-rpm motor shaft loosened the bolts that lock the rubber tire between its hubs. Motor torque and pump thrust loads accelerated the problem. Bolts and clamp rings broke away, the elastomer came loose, and the motor-side hub was left vibrating on the shaft. In some cases, part of the coupling broke away and tore into nearby components.
To protect other components, engineers added a steel guard around the coupling shafts. “The guard eliminated the side effects,” says Terry Williams, Customer Services Support Manager, “but it didn’t cure the problem.”
In less severe cases, where only the elastomer failed, lack of torque transmission rendered the deicing system inoperable. If you’re deicing in the middle of the airport and the coupling comes apart, all the fluid stays in the truck, not where it’s needed.
Though back-up trucks were available, planes sat idle until the crew replaced the disabled truck. It could take more than 3 hr to replace the coupling, and downtime could stretch to 6 hr.
Simon engineers turned to a compression- type elastomeric coupling better suited to the deicing system loading conditions. The Centaflex Series-A Model 2 from Lovejoy Inc., Downers Grove, Ill., consists of an elastomeric donut fastened to two steel hubs (one flanged) by hightensile fasteners, alternately arranged axially and radially, Figure 2.
Mr. Smith says “The tire coupling failures might have been avoided if its bolts could be tightened frequently,” he says. “But that kind of ongoing maintenance isn’t acceptable in an application like this.”
The compression coupling’s fastening system eliminates such concerns. Six threaded aluminum sockets are molded into the rubber element. Three of these sockets have radially positioned screws that tighten the rubber element against the cylindrical motor-side hub. The other three sockets contain axially oriented screws that anchor the rubber to the flange of the pump-side hub. The vibration-damping rubber cushions the fasteners so they resist being shaken loose. Also, the screw threads are coated with an adhesive that locks screw and socket together.
The alternating arrangement of fasteners preserves the motor-to-pump connection even if the elastomer fails. In such an event, the radial screws rotate until they contact the axial screws, causing a metal-to-metal interference that ensures uninterrupted power transmission until the elastomer can be conveniently replaced.
The mounting screws preload the rubber in compression, which reduces flexing under load. This extends the rubber service life by reducing internal heat build-up. With a 24,780 lb-in./rad dynamic torsional stiffness, the rubber damps vibration and shock loading without exerting harmful axial reaction forces on shafts and bearings.
The coupling used in the deicing system, Figure 3, accommodates up to 1,440 lb-in. nominal torque and 6,000 rpm.
“The new couplings have been a tremendous success,” says Mr. Williams. “In three years of production, we’ve seen less than 5% failures.” By contrast, he estimates that 40 to 50% of the tire couplings used in the first two years had failed.
The coupling guard remains as a safety precaution. Despite the confinement, the compression coupling’s design minimizes heat build-up that can reduce the elastomer’s life. And, because the coupling requires no routine maintenance, restricted access isn’t a concern.
Director of Operations Bill Dempsey points to a Chicago customer using 34 deicers. “Since switching to the new couplings, our availability there rose to 93%, which is tremendous for this industry.”