Are Old Lockers Better Than New?Posted in How To: Transmission Drivetrain on April 27, 2018
The need for traction began the first time the first engine turned the first wheel and it spun helplessly for the first time. We all know the cure…get more than one wheel providing traction. What may be a surprise is how much some of those early traction aids were like the products we all use today. Some of the old inventors and their inventions should be enshrined in the four-wheeling hall of fame because they impacted our sport and created products we still use today. Here are a few of the most pivotal innovations in the development of performance traction products that have led to the lockers you use in your rig today.
1913—M & S/Scurlock Worm-Gear Differential
In May of 1913, William F. Muehl applied for a patent on a cross-axis, spiral-gear differential, and it was granted in March of 1914. According to period reports, the first design dated back to 1911. Lewis Scurlock became involved, mostly on the business side, and they formed the M&S Gear Company of Kansas City, Missouri. The M&S diff appeared just in time for the legendary Jeffery Quad 2-ton 4x4 military truck, the first truck designed primarily as a military truck to military specifications, and also the first mass-produced American 4x4 with standard traction-enhancing differentials.
The contract with Jeffery was lucrative and continued into the early 1920s, even after Jeffery was bought by Nash. Eventually, Muehl left the company and Scurlock patented a different version of the diff, offering them under the Scurlock Differential name. The most notable Scurlock application was for the Ford Model T, but they were available for Chevrolet, Dodge, and many other cars, trucks, and tractors of the era. It’s seen in various forms into the ’30s. Overall, the M&S/Scurlock is the most historically significant of the very early traction aids, of which there were many.
1939—Thornton Automatic Locking Differential
The long-reigning “King of Lockers” is the Detroit, which has been known by other names, including Thornton Automatic Locking Differential and No-Spin. It all started in 1931, when Ray F. Thornton (1898–1971) debuted the Thornton Tandem Drive, a tandem rear axle conversion for medium and heavy trucks. One of the key features was an inter-axle differential, or compensator, between the axles to prevent tire scrubbing in turns. The compensator unit was developed into a locking differential for use inside axles and became available as early 1939, marketed separately as the Thornton Automatic Locking Differential.
World War II brought the Thornton locker into battle. During the war, Thornton became Detroit Automotive Products, so you see them referred to as both “Thorntons” and “Detroits” in this period. A complete list of locker applications in that period is not available; we know kits were produced for passenger cars and light trucks. International debuted their 2 1/2-ton 6x6 trucks in 1941, produced primarily for the Marine Corps and Navy—the M-5H-6, a high-mobility “Deuce and a Half” that featured Thornton lockers in the two back axles and the super-flexy Hendrickson rear suspension. A unit was also developed for the Dodge WC series 6x6 trucks.
After the war, the Detroit Automotive Products Company continued to market the No-Spin in larger trucks. In 1969, the No-Spin went mainstream. The applications were expanded to extensively cover the light-vehicle market, and those new applications wore the “Detroit Locker” name. The rest is history, and though the corporate ownership has changed several times, the Detroit Locker is still very much a pillar and mainstay in the recreational 4x4 market.
1955—Spicer-Thornton Limited Slip/Spicer Powr-Lok
Ray Thornton started work on a limited slip around 1952, and by 1955 he had a design that would be manufactured by Spicer as the Powr-Lok. The Powr-Lok would become the first commonly available limited slip—and the first with broad market coverage. Starting with Studebaker-Packard in 1956 (“Twin-Traction”) and Jeep in 1957, the Thornton unit would be snapped up by numerous manufacturers. Soon Chrysler (“Sure-Grip”) would be on board, then Rambler (“Twin Grip”), Pontiac (“Safe-T-Track), and Lincoln (“Directed Power Differential”). It would even be used in some GM cars under the “Positraction” name.
Many people don’t realize the first Powr-Loks used cone clutches. They had the familiar ramping device with Belleville springs to increase bias ratio according to available traction torque, but it acted on a pair of cone clutches rather than the more familiar plate clutches. By 1958, the plate clutch version had debuted and began to replace the cone clutch types in all applications. Enthusiasts in the gearhead community soon learned the Powr-Lok could be tuned mild or wild according to how the plates were stacked. The Powr-Lok was installed into OE vehicles well into the ’70s, when the milder-mannered and less expensive Trac-Lok debuted. The Powr-Lok design is still in production by various aftermarket companies.
1958—Gleasman Dual Drive/Gleason Torsen
Vernon Gleasman (1912–2004) isn’t a household name, but his contributions to four-wheeling are significant. In fact, they go beyond four-wheel drive into everyday motor transport and motorsports. In 1955, Gleasman submitted a patent for what was termed a positive-drive differential that used the worm gear principle in a way that had not yet been done. He’d been working on it since the late 1940s, and thinking about it since the 1930s.
As a young man in the logging industry, Gleasman observed the Walter and Scurlock differentials in operation. He eventually came to the conclusion they nibbled at using the worm gear principle, but didn’t fully utilize the concept. Gleasman realized the roadblocks were in the tooling, and he solved the problem by designing and building special tooling to grind the gears in a way many engineers said was impossible. In doing so, he created a differential that came close to differential perfection: a high-bias unit with easy differentiation and with considerable inherent strength. In other words—beefy, very effective, and with impeccable manners.
By the time the patent was granted in 1958, Gleasman was ready for business. He quit his job as an engineer at White Motor Company to form Dual Drive Differentials in Cleveland (soon known as Triple-D). He quickly landed a contract with Eaton Axle to supply Dual Drives for big Air Force tugs moving fully loaded B-52 bombers in all sorts of weather. From there, he fleshed out a list of popular applications and began producing them. Along the way, the Dual Drive ended up as popular with the street performance crowd as with the 4x4 crowd because it delivered traction in turns without any adverse handing effects. In fact, with a Dual Drive, lap times on a curvy course were faster than without.
Triple-D continued through the ’60s, ’70s, and into the ’80s, then getting on the ground floor with the AM General who was developing the High Mobility Multipurpose Wheeled Vehicle, the Humvee. Competing for a contract to supply traction aids, the Dual Drive significantly outperformed the others in contention. It became a vital part of the HMMWV performance equation and helped AM General win the contract, so it owns a slice of the Hummer legend.
Triple-D almost lost the deal when AM General realized they were actually a very small company. According to Keith Gleasman, son of Vernon and a part of the company at the time, Triple-D manufactured 9,000-10,000 units per year at this time, and AM General needed a guaranteed ready supply many times that. As part of the deal, AM General demanded they either gear up to produce many more units, and do it in a very short time, or sell the design to a company that could take over high-volume production. Vern Gleasman was 70 years old at the time and ultimately decided to sell. Gleason Corporation, best known for producing gear-grinding equipment, stepped up, hired some of Triple-D’s engineering staff, and hit the ground running. Gleason was also the only company at the time able to build the special tooling for cutting the gears for the Gleasman design. So it was a good match. The name Torsen was applied soon after, for torque-sensing differential, and the rest is history.