the case of the feverish final drive

The Problem

A manufacturer of vehicle drive assemblies was getting some complaints from their vehicle manufacturer customer regarding noisy units in the field. It seemed that a fair percentage of owners brought their vehicles into dealer service, complaining of a ‘whine’ in the rear, quite soon after taking delivery of their new vehicles. The whine was identified as gear noise emanating from the differential gearset. Further mutual investigation by the two manufacturers, which involved a fair amount of calls to the owners, concluded that this group had a very large percentage of the owners taking a multiple-hour, heavily loaded trip within a very short time after taking delivery. A typical example would be a pickup truck with dirt bikes in the bed, and a travel-trailer or boat / trailer attached. The trips also quite often consisted of fairly long runs at interstate speeds, as the owner and the family headed to their cabin in the woods.

The manufacturer of the drive assemblies identified the whine being a result of improper break-in, exacerbated by the resulting very high internal temperature of the unit, which resulted in the gear lubricant losing its load-bearing capability. The result was very accelerated gear wear, and the resulting gear noise.

The Investigation

We were asked by the client if we could offer a design which would lower the internal temperature to a level where the lubricant would have a better chance of maintaining its load-bearing capability, thereby allowing a much more gradual and proper break-in of the gearset. 

Our first task was to learn just how much heat was being created in this scenario, and we determined that the unit, as was presently in production, reached its maximum temperature while producing 24,000 BTU/hr (over 7 kilowatts of heat!). It was obvious to the design team that some sort of active (not passive) heat rejection system would be the only logical solution. The client, when apprised of this, added cost impact to their product as a prime project parameter as well.

A thorough research of all potential active systems was conducted, with two primary candidates showing the most promise. One was heat pipe technology, and the second was an air-to-oil radiator, utilizing the existing lubricant within the differential. Both designs were approved for prototyping, the units to be tested on an actual final drive, running on a dynamometer under the known loads.

The Design Challenge

The heat pipe concept was subcontracted to a house very familiar with the technology, and they delivered their prototype on time for comparison testing. The air-to-oil proposal had two primary components, one being the actual radiator itself, and the other being the methodology by which the oil would be moved from inside the differential to the radiator. The radiator itself was straight-forward; the design challenge was to develop a method to circulate the oil out of the differential, to the radiator, and back into the differential without creating any real cost impact. 

The Solution

We hit upon utilizing the large teeth on the ring gear as a ‘water wheel’ (for lack of a better description); what was modified in the overall differential was the shape of the rear stamped cover, which was reshaped to be much more enveloping of the ring gear, thereby minimizing the ‘spillage’ off the teeth. The oil was collected at the upper end of the teeth’s travel (again by a collection point stamped in the cover) and a simple hose connected that to the radiator, allowing gravity to handle the rest of the flow thru the radiator and back, via a return hose, to the bottom of the stamped cover.

Dynamometer testing showed that the air-to-oil design reduced the oil temperature to the desired level, outperformed the heat pipe, and it also was quite a bit lower cost as well. 

The client and the vehicle manufacturer decided not to add anything that would impact the vehicle overall cost to any degree, preferring to do further studies in lubrication, metallurgy, etc., in hopes of minimizing the whine issue without the application of a cooling system. 

This article was written by Tom Weisgerber, Vice President of Engineering for Kevin Kennedy Associates, who has over 25 years of experience in systems engineering. Tom can be contacted at 317-536-7009, or via email at TomW@KevinKennedyAssociates.com.