This is intended to provide a comparison between LOCK-N-STITCH
Inc.'s metal stitching products and other metal locking processes.
There are two parts to metal stitching: locks (or other
strengthening devices) and stitching pins for sealing.
Old Metalock design
The strength of a lock is determined by three things.
1. gripping strength of the lock
2. minimum width of the lock
3. tensile strength of the lock's material
This is the ability of
the lock to securely hold to both sides of the crack. The grip is
established by the difference between the major and minor width dimensions of
One of the LNS design locks
(L30) has a major dimension of 0.400" and a minor of 0.200".
This is a difference of 0.100" per side. The small lobes offer even
more grip by themselves with 0.025" per side.
A similar old metal lock device has a
0.246" major and 0.194" minor. This only has a 0.022"
difference per side. And this fastener has straight sections with no grip
between the lobes.
Another aspect of grip that is very
important is the manner in which the hole pattern is created. The LNS
hole patterns are created entirely by drilling. Our drill jigs create a
precision hole pattern. In order to prevent spreading pressure on the
crack, we have designed our locks to exert a pulling pressure on the two sides
of the crack. This is done by manufacturing the locks to be shorter than
the hole pattern. The difference is at the center of the center lobe
(which is installed right over the crack.) This slight pulling pressure
will prevent any spreading of the crack.
The hole pattern for an old metal lock device is created by
drilling spaced holes and then connecting those holes by chiseling out the
material in between. The chiseling method does not create a precision
pattern to receive the lock. Often the chisel can be more to one side than
the other, effectively destroying the grip on one side of the lock. When
the device is driven into the hole pattern, it usually does not fit very well
and must be peened to fit the hole. Some will claim the peening is done to
increase the strength of the lock, but, in essence, peening is done more to make
the lock fit.
One more aspect of grip is the the casting
itself. A repaired casting that is highly dependent upon the strength of
the repair can often exert cyclic strain on the repair. The exaggerated
lobe profile of the LNS design provides a significant amount of surface
area on the strain side of the lobes to resist stretching of the hole pattern.
Because of the design, the old metal lock fasteners have very
little resistance to cyclic strain.
LNS locks have an
excellent minimum width to maximum width ratio that maximizes strength.
Some lock designs utilize a hole pattern created completely by drilling a series
of overlapping holes with the same size drill bit. The problem with this method
is that the strength of the lock itself is diminished by reducing its minimum
width. This would increase grip, but would cause the lock to break instead
of slip in the hole pattern.
Tensile strength: Strength
is also derived from the tensile strength of the material the lock is made
of. LNS locks are made of 4130 steel and are heat-treated to
175,000 PSI tensile strength. The larger locks are created by wire EDM for
higher precision. Our locks have been used successfully in thousands of
applications. Some repairs have been in equipment subjected to very severe
applications such as salt water and heavy brine. We have had a few
opportunities to inspect our repairs years later and have always found there to
be more erosion of the base metal than of the locks or pins. We can also
make our locks out of stainless steel and bronze for special situations.
Most of the old metal lock processes make their locks from
Invar 36. With an annealed strength of only 65,000 PSI and a maximum cold
work strength of 104,000 PSI, this material barely offers a two to one ratio
over cast iron. The 36% nickel content in Invar 36 offers some chemical
and corrosion resistance, but reduces the amount of strength that can be placed
into the repair. Their primary reason for choosing Invar 36 is because of
its minimal coefficient of thermal expansion. As the operating temperature
of the casting increases, the length of the lock does not. This creates a
drawing effect on the two sides of the crack repair. However, what about
the width of the lock? A gap will form on the sides of the lock. And
the fact that the lock does not expand and contract with the casting causes
cyclic stressing of the lock and the hole pattern.
Here are a couple of additional comparison
points between LNS locks and the older metal lock devices. LNS
bottom locks are thinner. This is done to allow installation of a laminated stack
of locks that resist cracking better than a single lock. LNS locks
have greater overall strength than equivalent-sized metal lock devices.
(Example: LNS L20B = 6,650 PSI; their equivalent size = 4,035 PSI)
Additionally, when installing LNS locks, there is no need for
master locks or large pieces of steel to be set into the casting and then pinned
to create strength. Excavating cast iron for these types of strengthening
devices is very time consuming. Installing LNS locks is quicker,
easier, and stronger.
Tapered screw used by some Metalocking companies
LOCK-N-STITCH Castmaster stitching pin
The other component of metal stitching is the stitching
pins. Over the years many things have been used to replace the crack and
surrounding material. Some of those items have been: bolts (copper-
and zinc-plated); tapered plugs (both threaded and non-threaded); threaded rods;
screws; set screws; dowels; and just about anything else that could be stuffed,
hammered or screwed into the space. Over time, these items have been
called plugs, pins, and stitching pins. Stitching pins have evolved over
the past seventy years from simple threaded bolts through tapered pins to the
highly sophisticated threaded fasteners available today from LNS.
Until the innovations of LNS, starting in the late
1980's, metal stitching had several critical limitations:
Lack of speed: Installing tapered pins,
bolts and sundry other plugs is slow. The tapered taps used to thread the
installation holes must be hand-driven. The driving head of a tapered pin
or bolt or the remainder of the rod or all-thread must be cut off with a hacksaw
or a grinding wheel.
Difficulty of use: There is a lot of
guesswork involved in the installation of these items. The installer has
to guess how deeply to run the tap and how tightly to tighten the pin. If
the guess is wrong, there are new problems.
Inconsistent appearance: A tapered pin
usually leaves a flawed finish on machined surfaces. If the pin is
overtorqued, it will break off below the surface. And the other items,
such as bolts, rods and set screws, present challenges of their own.
Unpredictable seal: Because tapered pins
rely upon guesswork for tightening, it is common for them to be installed at
different torques. Because bolts, rods, screws and other items are not
precision installed, their pressures also vary. This can result in
loosening of previously installed pins and subsequent leaks.
Spreading pressure: Tapered stitching
pins, bolts, screws, rods, etc. all exert extreme spreading pressure on the
crack because they are simply wedges forced into the space.
Structural limitations: Repairs
attempted on complex contours, such as inside and outside corners, were very
difficult at best and impossible at worst. Taper pins (et. al.) require
significant strength in the surrounding material to contain their spreading
pressure (and locks require a fairly flat spot for installation). Cracks
found on or near outside or inside corners or near an edge can't be repaired
with bolts, tapered stitching pins or locks.
Here is a glimpse at the advantages of metal stitching now at
the fingertips of repair craftsmen because of LOCK-N-STITCH Inc.'s
Installing LNS can be done
quickly with pneumatic tools. Up to one inch of repair can be done in 5
minutes in 1/4" thick cast iron.
Ease of use:
The pins are designed with a
breakoff groove that causes each pin to break off at the same predetermined
Flawless finish: All LNS pins have
shoulders that set into the casting so the threads aren't exposed at the surface. Perfect metal-to-metal finishes are easy to
accomplish. Stress free, uniform repairs seal without the use of epoxies and coatings
required by inferior stitching products.
LNS pins and locks
create strength and seal over every linear inch of the repair.
Complex contours: Inside and outside
corners are now as easy as flat surfaces. C Series (CASTMASTERô)
stitching pins don't spread the crack, in fact they pull the sides of the crack
Links to pages about Locks and stitching
pins: L Series C