No matter how careful the manufacturer is to avoid problems,
customers will sometimes encounter problems installing nuts onto bolts.
Fasteners for manufacturing assembly are less likely to be returned due to
dented threads than MRO users. The reasons for this relate not only to the way
the fasteners are made, but also to the market's desire for these fasteners to
be easy to assemble. Manufacturing assembly allows the power wrench to continue
the locking operation simply through the starting face of the bolt or nut
teeth. The service technician will screw the nut onto the surface of the
workpiece by hand, and then use a hand wrench to complete the installation.
Neither condition is very practical, but there are several factors that affect
ease of use during assembly or disassembly, including "recessing of
threads" and "coating thickness".
Depending on the different manufacturing process and handling
methods, the chance of dents in the thread of the fastener during
manufacturing, loading and unloading, and transportation can range from 12
times to more than 20 times. Other additional factors include the size and
weight of the fastener. Of course, the thin threads of large-diameter fastener
products are most likely to be dented or flattened. There is also a chance of
thread sinking during packaging and shipping. Many fastener manufacturers use a
spring-loaded platform under the container to shorten the distance that the
bolt can pop out of the rolling machine to the ground. Other manufacturers use
shallow containers. Either way, soft bolts that have not yet been hardened fall
from the container of the bolt-making machine into other containers for
transport to the heat treatment furnace.
There are two common categories of heat treatment, including batch
heat treatment and conveyor belt heat treatment, both of which have advantages
and disadvantages. I did research in a facility using both types of heat
treatment and found that strength class 5 bolts had less dent in the threads
when processed through a batch oven. The fasteners are placed in a single large
container and then passed through a long heat treatment furnace for batch
processing without staggered displacement of the fasteners from each other.
Through the heat treatment of the conveyor belt, the soft fasteners will fall
on the conveyor belt, and then the parts will hit each other again. Heavy
equipment manufacturers often use black, uncoated fasteners because the
finished product is painted with corporate representatives to address the need
for corrosion protection. Due to general thread tolerances, most minor thread
dents will go unnoticed during assembly. However, unplated bolts are not easy
to store unless they are heavily oiled. Too much oil can make the fasteners
difficult to install and remove, but the bolts used in the process won't sit
still for long. Phosphate phosphate coating can be used to paint, providing
moderate corrosion protection in wet environments. The setup is very compact
and the interference between threads is negligible. The lubricating effect of
the coating assists assembly operations and reduces galling from heat-treated
scale. The automotive industry has specified a more organic dip-spin coating
method than electroplating. Basically all MRO fasteners are galvanized. No
matter what kind of coating is used, the extra thickness creates a press-fit
effect with the fitting part. This produces inconsistent assembly torque,
resulting in lower clamping loads.