While I have never owned or used one I have spent some considerable time looking at the design and I am not convinced the gearing is "fragile" per se. The basic construction is quite stout with several big honkin' 6 DP straight cut reduction spur gears. Those are some heavy gears capable of trasmitting significant loads!!! The units are however quite prone to gear teeth stripping failures likely caused by a poor shifting design that allows the reduction gears to disengage under load stripping the teeth in the process. Here is a picture of showing what the Howard reduction mechanism looks like. The picture is taken from what would be the top looking down. The heavy plate at the top would be bolted to the back of teh tractor transmssion.
The smaller spur gear at the center top is is the Howard input gear which is splined to the transmsiion output shaft. The compund gear that engages the input gear is one of two identical reduction gears - a second one is similalrly mounted on the underside of the input gear. The reduction gears are rigidly mounted between the nearly 3/4" thick case halves on stout ball bearings and 1" shafts. The large gear in the bottom center is the Howard output gear which is mounted on the pinion drive shaft and engages both of the smaller reduction spur gears. The input gear drives the larger spur gears on the reduction clusters which then drive the output gear via the smaller compound spur gears resulting in an overall reduction ratio of 3.5:1 at the rear axle.
The Howard output gear is splined to the pinion drive shaft and can be shifted into direct drive by sliding it upward in the picture. This disengages the external spur gear from both of the compund reduction gears and an internal gear cut into the face of the output gear slips over the input gear directly locking the pinion drive shaft to the transmission output shaft with a resulting 1:1 drive ratio. The compund reduction gears simply turn with no load when the gearbox is in direct drive.
Looking at the picture you can see the typical failure - the smaller reduction gear teeth and the mating teeth on the pinion drive gear are eroded along the inner edges. That pattern indicates to me that the teeth failed with the gears in partial mesh. Possibly from someone trying to shift them into engagenment with the tractor in motion. However I suspect a more likely cause is the shifter detent allowing the gears to come partially or fully out of mesh under a heavy load stripping the trailing edges of teh teeth in the process. So my take is the real design flaw is the shifter detent mechanism - not the actual gear sets.
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