Sealing flanges with room temperature vulcanising silicones has many
advantages but the preparation of the flange is critical to the success
of this method. Bob Orme describes an optimum flange design that involves
using an internal chamfer to create a wedge of cured product that flexes
with the joint
Engine designers face a number of dilemmas. For example, there is an
ongoing need to reduce weight to help fuel efficiency. Similarly,
vibration and noise levels must be cut. But, any engine weight decrease
can result in stiffness reduction. These considerations are compounded by
demands for 'zero' leakage of joints. Bearing ladder designs offer a
number of advantages, including an increase in engine stiffness - yet
with lighter weight, less noise and vibration and reduced machining and
casting costs. However, these designs require accurate tolerances of the
flange faces to ensure correct dimensions at the crankshaft journals.
Metal gaskets can be used, but they create shim between surfaces, require
manual assembly and have a relatively high cost. Liquid sealant between
flange surfaces is the solution. This allows metal-to-metal contact,
while increasing structural rigidity - and it can be applied robotically.
Where there is flange movement associated with lightweight designs, it is
best to use a room temperature vulcanising (RTV) silicone sealant. With
flexibility from 150 to 550%, a significantly high degree of joint
movement can be accommodated. Indeed, tests have revealed that movement
of 80(m can be tolerated. However, there are three conflicting
requirements for sealing with RTV silicone. First, the gap between
flanges needs to utilise fully the added flexibility of the cured
product. This creates a 'thickness' of flexible sealant that allows
relative movement of mating faces. Secondly, the joint must withstand
on-line pressure tests before the product has fully cured. If a gap
exists, the uncured RTV could be forced out when pressure is applied.
Thirdly, if metal-to-metal contact is not achieved, bolt relaxation might
occur. These 'conflicts', however, can be overcome by correct flange
design.
Japan's motor industry was among the first to adopt RTV silicones as the
preferred flange sealing method. Joints were generally rigid with close
bolt spacing, resulting in restricted movement. Some designs incorporated
grooves on the flange surface between bolts where there is limited
movement and a reduced need for sealant flexibility. The sealant is
applied into the groove, the joint closed and allowed to cure. But as RTV
silicone cures by contact with atmospheric moisture, there is a
protracted cure time for product deep within the groove. There is also
the risk that excess of sealant is applied, and when this excess is
squeezed, it can fall into the engine interior - potentially blocking
filters or oil ways.
Loctite's Global Engineering Centre in Germany has conducted extensive
testing of RTV silicones to determine the optimum flange design to meet
the dual requirements of on-line pressure test compatibility and induced
gap for flexibility. The solution involved using an internal chamfer to
create a wedge of cured product that flexes with the joint. There are six
benefits of the chamfer design: it creates an instant seal through
metal-to-metal contact; there is fast curing of exposed 'wedge' of
product; an ideal product thickness is produced; squeeze-out is
constrained by the chamfer; sealant consumption is low, and the surface
of the flange can be 'as cast'. The chamfer design was fatigue tested to
compare its performance with that of groove and metal-to-metal
configurations. The metal-to-metal joint failed after 10,000 cycles, the
groove flange design after 400,000, but the chamfer design remained leak
free after 10,000,000 cycles.
For high volume production, RTV silicone is best dispensed via a
seven-axis robotic syste