Engineers at the Maytag Corp. teamed up with a gear design consultant and a plastics manufacturer to develop a washing machine transmission that cuts noise and weight, and has fewer parts.
The new transmission incorporates plastic gears in a dual drive arrangement to save 13 lb and eliminate 42 parts compared with a conventional metal gearbox. it also reduces the noise compared to steel gears.
Switch to plastic
The company’s Herrin, Ill. plant has long manufactured washing machine transmissions with steel gears. Assembling them was complicated by clips, snap rings, and other fasteners that held parts in place. The steel gears generated much of the noise associated with washing machines. These transmissions typically weigh about 35 lb and need to be balanced by a heavy counterweight during the washer spin cycle.
Maytag engineer Jack Crain set out to design a quiet, inherently balanced drive that is easier to assemble and provides a washing action equal to or better than the previous transmission. He selected plastic gears for the new transmission, because they are inherently lighter and quieter. Injection molded plastic gears also offered the opportunity to consolidate, and thereby eliminate parts.
Dual drive
Achieving the benefits of plastic gears required a completely new transmission design. Mr. Crain explains, “The load carrying capability of plastic gears compared to steel wasn’t sufficient for a conventional drive. To accommodate this lower strength required a dual power path.”
The new dual-path drive, with two identical sets of spur gears, is located in a diecast metal housing above a 3/4-hp motor and a wrap spring clutch that engages the input pinion of the gear drive. The input pinion gear turns two cluster gears — each one consisting of two gears of different diameters molded in one piece, Figure 1. Each cluster gear turns a second- stage crank gear, which drives a rack in a reciprocating motion. The two racks, in turn, drive a floating output pinion on the agitator shaft (not shown) that creates the washing action.
Peak washing action calls for the agitator to rotate 155-deg in one direction, then the opposite direction, with each pair of rotations occurring 92 times per minute. Each stroke of the rack, which produces one 155-deg rotation, occurs in three phases: smooth acceleration from 0 to maximum speed, smooth deceleration from maximum speed to 0 speed, followed by a dwell. This type of motion gently reverses the agitator rotation, thereby reducing the shearing action on the clothes associated with most washers.
The two-stage spur gear drive is inherently balanced because all of its rotating parts are symmetrical about its center. The gears are molded with timing marks on their sides, so assembly workers align the marks on each set of meshing gears. This alignment ensures correct positioning of the racks and synchronization of the two power paths.
Tight tolerances
Once the basic design was established, staff engineer Zan Smith of Hoechst Technical Polymers (HTP) and gear design consultant Ken Gitchell of the Universal Technical Systems (UTS) Inc., refined the gear design to the following dimensions:
• Input pinion: 1.4-in. diam and 17 diametral pitch (dp).
• Cluster gears: first gear 3.4-in. diam and 17 dp, second gear 1.3-in. diam and 14 dp.
• Second stage crank gears: 3.5-in. diam and 14 dp.
Pitch line velocity and noise reduction requirements called for gears molded to American Gear Manufacturers Association (AGMA) Q7 quality standards, which are more stringent than for most plastic gears. For example, this standard specifies a tooth-to-tooth composite tolerance of 0.0018 in. and a total composite tolerance of 0.0046 in. for the input pinion. Such tolerances are particularly demanding for injection molded gears.
Using a software program for designing plastic gears, the engineers also calculated the gear loads and the allowable dimensional changes under different environmental conditions, to serve as a basis for selecting the plastic material.
Material properties
Because plastic deflects more under load than steel, the gear material needs sufficient stiffness and creep resistance to prevent excessive gear tooth deflection. The material also must maintain dimensional accuracy despite heat generated by the clutch. “Heat was our enemy in developing the design,” acknowledges Mr. Crain. Normal operating temperatures are up to 135 F, but rigorous test conditions require operation up to 190 F.
Heat generated by the clutch necessitated oil bath lubrication for both the clutch and gears. The gears consequently must maintain their strength and dimensional accuracy despite long-term exposure to oil.
After evaluating several plastics, engineers selected Celcon acetal copolymer. Except for the input pinion, the spur gears are made from a 25% fiberglass reinforced version of acetal to obtain sufficient strength to carry the loads and control tooth deflection.
Less heavily loaded than the other gears, the input pinion has two radial springs molded into its sides, Figure 2, which engage cam lugs in the transmission housing. The springs compress and ride over the lugs when the agitator oscillates. But when the motor reverses to stop agitation and start the spin cycle, the springs expand and engage the lugs, causing the washer basket to spin. Because of lower tooth loads, plus the resilience required in the springs, engineers chose a non-reinforced acetal copolymer for the input pinion.
With all of the plastic gears in the transmission, only the gear racks are made with powder metal.
Quieter and lighter
Though Maytag engineers won’t disclose the degree of noise reduction, the new plastic gear train is unquestionably quiet, according to Mr. Crain. “With the polymer gears, we’ve reduced gear noise to the level where most people can’t hear it at all.” This is because the gear noise is now less than that of other washer components.
The impact on weight and assembly steps has been equally impressive. With the transmission 13-lb lighter than its steel predecessor, workers can assemble the new drive and move it by hand without counterbalanced lifting aids. “Reducing the number of parts made the transmission a lot easier to assemble, and more reliable,” says Mr. Crain. “There’s far less chance now of an assembly line error.”
Engineers eliminated most of the parts by combining multiple functions in a single injection molded part. For example, the cluster gear combines two gears in one, and the input pinion combines a gear and two springs, Figure 2.
According to Mr. Crain, “We’ve seen far less wear on the plastic gears than on the steel gears in our previous transmission.” Further, test drives have demonstrated a 10 to 15% reduction in heat rise, due to improved efficiency.
After the plastic gears passed laboratory life tests, Maytag put them into production- line washing machines in September, 1996. To date, the new units have performed satisfactorily with no surprises.