A sealant for every joint?

Sealing threads and flanges to prevent leakages is an essential to ensure systems function reliably. Numerous design solutions are available and most perform adequately. However, selection should not be based on engineering criteria alone.

Analysis of economic aspects is also important, including expenditure incurred for gasket inventories.

The development of up-to-date sealing systems has made tremendous progress during the past few years. For example, cars of the late seventies or early eighties, often left an oil stain on the garage floor underneath the car which always got worse as the car grew older. Leakages were not restricted to the oil pan – lubricants also escaped from other joints on the engine, transmission and axles.

If oil losses were too significant or went unnoticed for too long, damage could be caused to bearings or pistons. This is no longer a problem with today’s engines, although oil pressure in drive trains is much higher than in the old-generation models, making lubrication and sealing even more important.

In the past, pre-cut gaskets made of paper, plastic or cork, as well as sectional rubber gaskets were commonly used in vehicle building. These gaskets had to be designed and manufactured specifically for each application. They were tailored to the respective flange designs and to the quality of new parts. In ideal conditions they achieved long-term leak-proof sealing.

In the real world, all flanges which are designed for transmitting power or undergo major fluctuations in temperature, are subject to micro-movements. These result in slight relaxation of the gasket which in turn causes a loss of bolt pre-torque. Whenever a loss of bolt pre-torque occurs, surface pressure between the flange sealing faces is reduced, permitting additional relative motion between parts. Leakage of the material to be contained by the gasket is the inevitable consequence. But even if threadlocking devices are used which prevent loosening of the bolts, solid gaskets will not obtain a perfect seal. Low-viscosity engine oils may creep through the surface roughness of flange faces or small imperfections in the flange surface to cause oil stains.

Liquid anaerobic gaskets on the other hand achieve metal-to-metal contact between the flange faces and completely fill the surface, effectively preventing any oil from escaping through these potential leak paths. In addition to sealing, anaerobic gaskets bond the two flange faces, producing a unitised assembly which offers increased structural integrity and strength.

The cured sealant is resistant to most engine fluids and coolants used in engines, transmissions and axles.

The basic prerequisite for successfully making gaskets with liquid sealants is a clean surface free of oil and grease. For this reason, all sealing faces should be pretreated with suitable cleaners. The sealant may be applied by screen printing, by robot, or manually. It is sufficient to coat only one of the flanges with sealant, then assemble the parts and tighten the bolts.

Leak tests can be performed at low pressures after a short period. Within 24 hours, the products will be fully cured into a plastic providing a permanent, united flange assembly. For sealing flanges which have to be dismantled for servicing, the new Loctite 5203 offers the benefit of easy disassembly of flanges. A plastic spatula can be used to remove the product from the surfaces without any problems and without damaging the flanges, significantly reducing the cleaning effort required before re-assembly.

The prevailing trend towards further weight reduction inevitably leads to a move away from the commonly used castings, to thin-walled aluminium or magnesium parts, punched or deep-drawn sheet metal and plastic parts being used for drive trains.

Anaerobic sealants are not suitable for these materials, as well as for narrow flanges because, on the one hand, the prerequisites for curing – exclusion of oxygen and metal contact – are missing, and on the other hand there are also wider tolerances and gaps to be accommodated.

For these reasons, flexible, moisture-curing gaskets are used. These products are capable of sealing through gaps of >0.3mm and will harden to form flexible gaskets. In addition, they exhibit superior adhesion. Examples of this technology are the Loctite grades 5900 and 5910. Product 5900 is used primarily in line production processes with robots for precise dispensing onto the components, while Loctite 5910, a lower-viscosity version, is used mainly for repair and maintenance jobs. Typical applications include oil pan gasketing, sealing of end caps, or of components subject to high vibration loading. Differential thermal expansion, eg, between aluminium and steel, or plastics and metal, is no problem for the flexible Loctite sealants 5900 and 5910.

SEALING THE REST

Threaded assemblies are a permanent fixture in machine building. They are easy to assemble and disassemble, they can transmit power, they can be standardized and – they can prevent leakage of operating fluids if joints are assembled with a sealant. A look around any bathroom, kitchen, boiler room or at machinery with high-pressure hydraulics, threaded assemblies will be noticeable, requiring tight, leakproof joints within the pipework.

For sanitary plumbing, this is usually achieved by three different methods: either with hemp and cement, Teflon tape, or with a rubber ring. However, all three methods have drawbacks.

Problems may be encountered if the material to be contained is not moist, but dry like gas. Teflon tape is hidden inside the threaded joint and will also obtain good sealing performance for dry substances, but where surfaces are smooth it will be difficult to apply this material to the threads. Finally, when the parts are assembled, the seal on threaded joints made with Teflon tape is frequently broken and may leak when fittings have to be backed off for alignment.

Rubber rings, like solid pre-cut gaskets, require large inventories of different diameters. Joints have to be assembled with care to avoid tearing and destruction of the sealing face. All these sealing systems have one thing in common: sealing of the joint is time-consuming and requires dexterity and a delicate touch.

Where threaded joints have to be made in high pressure hydraulic systems, liquid sealants are used almost exclusively to obtain a unitised assembly. Liquid thread sealants, just like liquid flange gaskets, create a homogeneous, structural assembly. In addition, the complete thread area is used for sealing – contrary to rubber rings that seal only in the specific area of contact. A further benefit is the fact that the visual appearance of threaded assemblies sealed with liquid sealants is perfect, and there is no problem aligning the parts within 15min of assembly.

Loctite offers three core products for this application area. Thread sealant 542 has been designed specifically for hydraulic and pneumatic fittings. Product 577 is the preferred option for heating and sanitary applications. Being anaerobic thread sealants, these two products are suitable for metal threads only. The product of choice for preventing leakages on plastic-to-plastic or metal-to-plastic assemblies is Loctite 5331, a permanently flexible thread sealant.

Loctite offers also a pre-applied thread sealing solution with Product 5061. Threaded parts are supplied already coated with this product. There is no risk of forgetting the sealant, and the variety of parts held on stock can be reduced significantly.

LOCTITE UK Tel: 01707 358800