tech
What is a
Slip Yoke Eliminator and Why Do You Need It?
Some information provided by
JB Conversions
According to JB Conversions
there are 3 main reasons to install the Heavy Duty Slip Yoke
Eliminator kit in your Jeep:
1) Strength:
With the exception of the
stock output shaft found in the "J" model, the NP231 can
handle considerable abuse without exhibiting any signs of
distress. The planetary gear assembly, drive sprockets, and
chain in the 231 are very strong and rarely have problems.
When there is a problem, low oil level is usually the cause
of failure. The original 27 spline output shaft in the 231
is the only real weak point in the entire unit. The stock
output shaft features a spiral cut speedometer gear just
past the rear output bearing. The minor diameter of the
speedometer gear teeth is fairly small. When the shaft fails
it will almost always break in this area.
2) Lubrication:
The rear output "slip-yoke"
design of the NP231 is notorious for leaking. The rear
driveshaft yoke used with a slip-yoke style transfer case is
designed to slide back and forth across the splines of the
output shaft. Under normal driving conditions and vehicle
payload, the slip yoke will remain within a narrow range of
movement across the splines and seal surface. The surface
area past this narrow margin is exposed to the elements and
will form a ruff surface due to rust and dirt. Once the
vehicle becomes loaded down with trail gear, or the
suspension is cycled heavily, the slip yoke will travel even
more therefore dragging the corroded surface across the
rubber output seal. This is where the leaks start. Over
time, a low oil level could be reached within the transfer
case and the result could be catastrophic failure.
Another problem with the slip
yoke design is that the rear driveshaft yoke cannot be
removed from the vehicle without loss of transfer case
fluid. Broken u-joints can often require the vehicle to be
driven back to civilization on the front shaft only. Without
a fixed yoke on the rear output, this task would be
impossible without experiencing major transfer case damage
due to loss of oil.
Kit Upgrade: The HD slip yoke
kit utilizes a standard Dana Spicer end yoke. This "bolt-on"
yoke eliminates the leak prone slip yoke entirely. With the
addition of the fixed yoke, the rear output seal runs in one
location on the yoke therefore seal life is maximized.
3) Driveline angles:
On any short wheel base
vehicle, driveline angle is critical and easily influenced
with the addition of a suspension lift. Typically, Jeep
Wranglers come factory equipped with a rear driveshaft that
is 13.5" or less. With a running length this short, every
inch of added suspension lift has a drastic effect on the
angle of the driveline.
Kit Upgrade: The only way to
DECREASE the working angle of the U-joint is to lengthen the
driveshaft. Once installed, the HD yoke kit will add 4.5" (YJ's)
to 6.5" (TJ's) of length to the rear driveline therefore
flattening the overall slope of the shaft.
Gains in driveshaft length
are based on measurements taken from the center-of-u-joint
on the original driveshaft to the front of the case. Be
careful in buying a slip-yoke kit based on the claimed
length "number" in inches. It is better to look at the
design of the new rear housing in the fixed yoke kit as
opposed to that of the rear housing of the stock tcase.
Remember this; the stock NP231 has an output bearing at the
rear of the speedo housing as does the new slip-yoke
eliminator kit. These bearings are within 0.25" of the same
location as measured from the front of the case. The
shortest design for a yoke kit would be to place the yoke
immediately after the rear bearing. The JB kit does exactly
that. Anything shorter would require a shorter end yoke or
more shallow rear housing and that is not possible due to
the size of the speedometer gear bore. What this means is
that the JB kit is designed to be the shortest overall
length possible regardless of where you measure from.
After the installation of the
JB kit, the improved operating angles of the u-joints will
greatly increase the life of the driveline. The new HD yoke
kit is shipped with a C.V. compatible end yoke and requires
a 1310 series u-joint. Other end yokes are available to
accommodate larger u-joints however the 1310 is the
preferred joint.
See more
Transfer
Case Upgrades, Modifications, and Options |