Does not contaminate hydraulic systems because it stays in liquid form until air is excluded from the joint
It can be difficult to use
The parts need to be cleaned and primed before use with an evaporative solvent, which is an extra operation. This creates a problem when adaptors need to be fitted to a system which has already been in use.
Tapered Thread Orientation Issues
Another shortcoming of tapered pipe threads is that they often make installations difficult.
Consider the situation below.
The filter bowl needs to be facing the bottom, but problems arise when the bowl is facing down, as the tapered thread may not be sufficiently tight in that position.
This is the cause of many leaks with tapered threads.
Port Damage with Aluminium Parts
Some manifolds are made of aluminium, which is softer than the steel that the adaptors are made from.
The aluminium deforms and swells over time, eventually causing fresh leaks to emerge.
Port Damage with Cast Iron Parts
The valve below is made of cast iron. Cast iron is very commonly used in hydraulic couplings and adaptors.
Cast iron is brittle, and it cracks easily when stressed. Tapered threads push out radially when they are tightened, and if they are screwed in too tightly, will crack the port.
The valve on the left has been designed to accept tapered threads, and as a consequence, the valve body has been reinforced around the ports, using excessive thickness.
This makes the component bigger, heavier, ugly and more expensive.
Issues With Joining Tapered Threads to Hydraulic Tube
The biggest problem that tapered pipe threads have is that they cannot accept tube unless the tube is welded to the part.
The photo on the left shows a RYCO S53 adaptor. It has had a tube silver soldered to it.
Hydraulic hose is necessary and common in hydraulic systems. Silver soldering components require specialised equipment, skilled labour and time.
Poor quality workmanship is also a safety concern. For this reasons, documents such as MDG41 suggest that silver soldered joints should not be used.
Overview of Tapered Threads
Tapered pipe threads have a number of disadvantages that need to be considered when using them to connect up hydraulic systems:
They are the earliest form of hydraulic connector
They require thread sealant
They can be difficult to orientate
They can cause damage to threaded ports
They cannot be adapted to tube
The main advantage of tapered pipe threads is that they are very strong mechanically (they have a high shear strength).
This is because they seal on the thread only, and tapered threads are twice as long as other hydraulic thread forms such as JIC or UNO.
This is why they are used in high pressure hydraulic systems such as jacking systems.
Development of Tapered Threads
As hydraulic systems evolved, so did the connector types used with them.
One early development was that a sealing face was added to tapered threads.
This sealing face was first added to British Standard Pipe Threads (BSPT) and later used on many other connector forms.
Common Thread Pipe Forms
These are some of the most common forms of tapered pipe threads: