FIGURE 1. Split, star, and
FIGURE 2. Nylon and all-metal lock nuts.
Liquid and solid
Unfortunately for us, all these
causes are abundant in combat
bots! Here are the various ways to
deal with them.
Spring washers are the oldest
and least reliable way to secure a
bolt. They all work by adding
friction and tension between the
bolt and substrate. Unfortunately, it
only takes from 0.25 to two
revolutions of the bolt before the
spring washer loses its tension and
becomes useless. There are many
variations with different benefits.
Bellville washers are conical
washers used for maintaining a
uniform tension under a fastener,
and do not provide any real locking
function. Until they are completely
flattened, they act as a spring; their
main use in bots is as clutch springs
or as shock absorbers.
Wedge washers have two
parts that act as a ratchet and
require more force to release than
to tighten. They are probably the
most effective type of locking
washer, but have the drawbacks of
extra weight, volume, and extra
parts to lose.
Star washers have a number of
sharp edges that provide some
locking action, but cause surface
damage. Their main use is on lower
stress applications such as electrical
connections where a liquid
thread locker is unsuitable.
Split washers have been
widely discredited as useless.
Both NASA and military tests
show that when compressed,
they act the same as plain
flat washers and have little
Lock nuts are a step up from
spring washers with several important
advantages. There is one less part to
lose and, most importantly, the locking
action does not rely on tension
between the nut and the frame to stay
tight. This way, they resist loosening
right up to the point where they come
off the end of the screw. The most
common lock nuts have a nylon insert
and are effective up to about 250°F.
The all-metal variety is more expensive
and works at high temperatures —
this type is ideal for flamethrower bots.
The only real problem with lock
nuts is that they only work where
you have a through hole or an
externally threaded part.
Liquid Thread Locking
The best known brand is
Loctite. However, there are others
and you can even use hot-melt glue
in an emergency. The pros and cons
of liquid thread lockers are:
• One size fits all. You will not need
to stock different sizes and types
of washers and lock nuts. At
most, you will need two or three
different grades of Loctite.
may find other builders lining up
to borrow it.
• Loctite needs time to cure,
making it less suitable for screws
that are constantly removed
• Loctite is an anaerobic glue,
which means it cures in the
absence of air. Loctite stays liquid
until you screw the parts together,
after which it will begin to
harden. All varieties need some
curing time to reach full strength,
as shown in Table 1. Allow at
least enough time for 50%
strength to be reached.
• Loctite cure times are at 70°F with
steel parts. Raising the
temperature above 100°F will
shorten cure times, while using
less reactive metals like stainless
steel will lengthen curing time and
give lower holding power.
• Loctite products all lose strength
at higher temperatures; 243 loses
50% of its holding strength at
210°F, while 262 is down to 50%
strength at 300°F.
0.75 (0.25 @
0.2 (0.1 @
242 Medium Medium 300° F
243 Medium Medium X X 300°F
246 Medium Fast 450° F
Medium Medium - fast 300° F
262 High Slow
266 High Fast X X
268 High Slow
2760 Very high Medium X
TABLE 2. Loctite selection chart.
SERVO 04.2010 31