THE LINERLOCK -- RIGHT FROM THE SOURCE Michael Walker's invention and development of the LinerlockTM by Bernard Levine (c)1997 - for Knives Illustrated The "Linerlock" knife is now so familiar that it is easy to forget that both the knife and the name are relatively recent inventions. Michael Walker made the first modern Linerlock in 1980, and he registered the name Linerlock as a trademark in 1989. Since the mid 1980s, dozens of hand knifemakers and factory knife manufacturers have made locking liner type knives inspired by Walker's designs, although very few of them fully understand either the advantages or the limitations of this mechanism. The best way to understand the Linerlock is to look back at how Walker developed it. THE EARLY DAYS Mike Walker began to make knives early in 1980. One of his first customers was a collector and dealer in Red River, New Mexico, named Don Buchanan. Mike made ten fixed blade knives for Buchanan. Don asked Mike for sheaths to go with these knives. Mike made those leather scabbards reluctantly, then announced that he hated making sheaths. So Don said, "Make folders." Mike did. He made slip joints. He made lockbacks like the factory folding hunters then on the market. He made mid-locks with mechanisms copied from antique folders. But he was not satisfied with any of these. Walker envisioned an improved folder that would do away with what he saw as the many limitations of conventional lockbacks. First, he would design a knife that the user could open and close safely and easily with one hand, without having to change one's grip, or rotate the knife in one's hand. Second, his new knife would do away with the sharp "back square" of the conventional pocketknife blade. When a conventional blade is closed, its back corner sticks out, and can snag the user's clothing. In some folders the back square is enclosed by extended bolsters, but this can compromise the shape of the handle. Mike envisioned changing the basic geometry of the folder, in order to eliminate the problem entirely. Third, and most subtle, his knife would be self-adjusting for wear. Other innovative folders of this period, notably the Paul knife by Paul Poehlmann (patented 1976), were very strong and very sleek, but they required careful adjustment of set screws to keep their blades from working loose. THE LOCKING LINER Mike was familiar with the old locking liner design patented by Watson & Chadwick in 1906 for Cattaraugus. Used first on traditional folding hunters, this mechanism became standard on electricians' pocketknives, and was also used on Cub Scout knives. In this design, the liner projects above the handle, and it is split lengthwise, alongside the pivot pin. The side of its narrow tip engages the front edge of the tang when the locked blade is open. Mike noted that only a thin extension of the liner could be used as the lock in the Watson & Chadwick design. This was because most of the liner had to engage the pivot pin, in order to hold the knife together against the tension of the backspring. The result is that this type of lock is inherently weak. Mike went back to first principles. He realized that if spring tension and lock-up could be provided by a liner alone, he would be able to dispense with the backspring entirely. With the back spring gone, he could then have the end of the liner cut-out engage the bottom end of the tang, making for a much stronger and more positive lock. Indeed it would be nearly as strong as the old Marble's Safety folder (patented in 1902), while dispensing with that knife's long, awkward, and fragile fold-up extension guard (the folded guard serves as that knife's lock when the blade is opened). STRONG AND SECURE As it worked out, Mike had not anticipated just how strong his new lock would be. About 1984 I helped to run side-by-side destruction tests of all the types of locking folders available at that time. Each test involved securing the handle of the knife without blocking the movement of its blade or spring; then sliding a one-foot pipe over the open blade (which was oriented edge downward), to serve as a lever-arm; and finally hanging weights from the free end of the pipe until the lock failed. Name-brand conventional factory lockbacks failed at between 5 and 7 foot pounds (except for one that failed with just the weight of the pipe). A Paul button-lock knife proved to be more than twice as strong as the best of the conventional lockbacks. But a Walker Linerlock was nearly four times as strong as the lockbacks. What's more, when Walker's Locker did finally fail, it failed in the open position. Instead of closing suddenly upon failure, as all the other knives did, it seized up and became a "fixed" blade. SELF-ADJUSTMENT This strength turned out to be a fringe benefit of Walker's self-adjusting design. He based this design upon the simplest of all mechanisms, the inclined plane, or wedge. The end of the tang is slightly beveled. The end of the liner is not (although it can be, as long as the angles do not match). Both parts must be hard. When the blade is opened all the way, the liner passes the inner edge of the tang, but it is stopped before it passes the outer edge. The liner's leading edge bears on the beveled end of the tang. If the pivot joint loosens over time, the point of engagement of the lock-up moves further along the bevel, so it continues to lock up tight. In the destruction test, when we applied an extreme load to the blade of Walker's Linerlock, the free end of the locking liner moved all the way past the end of the tang, and wedged itself between the blade and the fixed liner. Mike was later able to disassemble and repair this test knife, and today it is (almost) as good as new. In his first Linerlocks (he was not calling them this yet), Walker made the liners out of spring-tempered 440-C blade steel (he did, and still does, his own heat treating). The lock-ups were not yet the full width of the tang -- Michael changed this after the destruction tests, to make his knives even stronger. The thick 440-C liners of those early versions applied so much spring pressure to the blades that no other mechanism was required to retain the blades in the closed position. But when Mike began to experiment with lighter gauge liners, he realized that a separate element would be needed to perform this function, which is performed by the backspring in conventional knives. In 1984 Mike began to incorporate a ball detent in the frames of his Linerlocks, allowing the liner to be dedicated totally to lock-up in the open position, while the ball detent held the folded blade closed. TITANIUM These new lighter gauge liners were made out of titanium alloy. Titanium has many features that make it especially suitable for this application. - Titanium has a high strength to weight ratio. - Titanium has superb spring retention qualities, without the necessity of any heat treatment. A titanium spring will recover from a severe load that would permanently deform a steel spring of the same cross-section. - Titanium galls to other metals -- it seizes to them, rather than slipping past them, when they are rubbed together under tension. This makes titanium useless for moving parts, but ideal for parts that are meant to seize, such as the end of a liner engaging the end of the tang of a folding knife blade. - Titanium can be electrolytically toned to a wide range of attractive colors. Michael and Patricia Walker pioneered the application of this technique to knives. In fact Patricia Walker was the first artist to engrave and anodize titanium, both on her husband's knives, and on her own jewelry and artwork. ADVANTAGES AND LIMITATIONS Walker's Linerlock mechanism is flexible and forgiving in many ways. In the 1980s Mike would go to shows with a box full of unfinished blades that he had ground freehand in all sorts of shapes. Customers would pick out ones they liked, and Mike would then make knives around these blades, without any need for the precise patterns that burden the makers of conventional lockbacks and slipjoints. However, one aspect of the Linerlock is not forgiving at all. This is the bevel at the end of the tang, on which the end of the locking liner bears. If this angle is too acute, the liner will slip and the lock will fail. If the angle is too obtuse, the liner will stick, and the blade will be difficult or impossible to close. Mike emphasizes that there is no single correct angle for this bevel, as some writers have mistakenly claimed. Rather it must be determined for each knife. The optimal angle is a function of the blade and liner materials, of the spring tension of the liner, and most important of all of the overall length of the knife. The free end of the liner moves in an arc of a circle, and the length of the knife determines the radius of this circle. LINERLOCKS TODAY Mike Walker rarely makes Linerlocks any more. He has licensed the name, and various aspects of the mechanism (including the patented safety latches recently developed jointly by Walker and Ron Lake), to a few other makers and manufacturers. On his own current knives he uses some of the dozens of other locking mechanisms that he has invented over the years. Mike is flattered that so many makers and manufacturers use his invention, though he is disappointed that most of them fail to grasp all the subtleties of the Linerlock mechanism. Because of this, most of their knives lack the strength and smoothness of Walker's own. And Mike is angry at certain pompous Johnny-come-lately makers who attempt to claim credit for his inventions and his designs. One shameless maker is today receiving royalties for a design that Walker created two years before that particular maker assembled his first knife. But Mike never patented his original mechanism or his early designs, so this sort of copying is now water over the dam. However Walker's trademark rights are another story. Mike lets his lawyers deal with any makers or manufacturers who have the temerity to use his "Linerlock" trademark without his formal written permission. *** END ***