Page 107 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 2 Common processes and techniques

KEY POINT

Pipe gain
When bending copper
tube using a bending
machine, the tube
appears to gain length.
This is called pipe gain
and we have to take
it into account when
precision bending. The
pipe gain on a 90° bend
is 1.5 times the diameter
of the pipe. For instance,
if using 15 mm tube,
p Figure 2.14 A passover bend the pipe gain will be
22.5 mm. Let’s say we
IMPROVE YOUR MATHS have to put a 90° bend
To find out how much tube is used in a machine bend, we have to know the on a piece of 22 mm
radius of the bend. Bending machines usually bend at a radius of four times the copper tube so that the
diameter of the tube. So, for a 90° bend on a bending machine using 15 mm finished measurements
pipe, the radius of the bend will be 60 mm. If we use the following formula, we are 150 mm end to
can find the length of the bend: centre and 250 mm end
to centre. The length
Radius × 1.57 = Length of pipe of pipe needed appears
Therefore: to be 400 mm but,
because the bend cuts
60 × 1.57 = 94.2 mm
the corner, we can
So, a bend with a radius of 60 mm using 15 mm pipe uses 94.2 mm of pipe. deduct a certain amount
This we can round up to 95 mm. of pipe. If pipe gain is
Now attempt these examples: 1.5 times the diameter
1 What is the length of the bend of 15 mm tube with a radius of six diameters and the diameter is
22 mm, we can deduct
of tube? 33 mm, so the actual
2 What is the length of the bend of 22 mm tube with a radius of four diameters pipe length needed for
of tube? the bend is 367 mm.
3 What is the length of the bend of 22 mm tube with a radius of five diameters Pipe gain occurs only with
of tube? 90° bends (Figure 2.15).

Bending copper tube using a bending spring
Bending springs are used to support the walls of the tubes against collapse
while the bend is being formed. The British Standard for bending springs is
BS 5431:1976 and they are available for copper grades R220 and R250. It is
important that the correct-sized spring is used or wrinkling and even snapping
of the tube may occur. p Figure 2.15 Pipe gain occurs
only with 90° bends
INDUSTRY TIP

As a rule, the bend radius should be four to five diameters of the tube. This is slightly more
than for a bending machine. However, choosing a four-diameter bend helps in simplifying
the marking-out process.

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