DESIGN OF TURNBUCKLE

Problem Specification

It is required to design turnbuckle for a connecting the tie rods in the roof truss. The maximum pull in the tie rods is 75KN.

Construction

The construction of the turnbuckle is shown in Fig. 01. It consists of a central portion called coupler and two rods. One has right hand threads while the other rod has left hand threads. The threaded portions of the rods are screwed to the coupler at the two ends. As the central coupler rotates, the rods are either pulled together or pushed apart depending upon the direction of the rotation of the coupler. The outer portion of the coupler is given hexagonal shape so that it can be rotated by means of a spanner. Sometimes a hole is provided in the coupler as indicated by a dotted circle in the figure. Instead of using a spanner, a tommy bar is inserted in this hole rotate the coupler. The turnbuckle is used for connecting two rods which are in tension and which require slight adjustment in length during the assembly. Some of its applications are as follows:

1. To tighten the members of the roof truss;2. To tighten the cables or the stay ropes of electric distribution poles; and3. To connect the tie rod to the jib in case of jib-cranes.

Fig. 01 Turnbuckle

Selection of Materials

Coupler has relatively complex shape and the economic methods to make the coupler in casting.

Casting reduces number of machining operations. Grey cast iron of grade FG 200( =200Pa) is selected

as the material for the coupler. The rods are subjected to tensile force and torsional moment. From strength

considerations, plain carbon steel of grade 30C8 ( =400 Pa) is selected for the rods.

General Considerations

Many times, turnbuckle is subjected to rough handling while in use. Sometimes, workers use a pipe to increase the length of the spanner and tighten the rods. Some workers even use a hammer to apply force on the spanner. To account for this mis-use, a higher factor of safety of 5 is used in present design.

The coupler and the rods are provided with ISO metric coarse threads. Coarse Threads are preferred because of the following advantages:

Coarse threads are easier to cut than fine threads; They are less likely to seize during tightening; and There is more even stress distribution.

The rods are tightened by applying force on the wrench handle and rotating the hexagonal coupler. The expression for torque required to tighten the rod with specific tension P can be derived by suitable modification of the equation derived for the trapezoidal threads. The torque required to overcome thread friction in case of trapezoidal threads is given by Equation.

=

For ISO metric screw threads,

Where, d is the normal diameter of the threads. The coefficient of friction varies from 0.12 to 0.2 depending upon the surface finish and accuracy of the thread profile and lubricantion. Assuming,

and substituting above values in Eq.

=

Or =0.098Pd

The above expression is used to find out torsional moment at each end of the coupler.

DESIGN FOR RODS

The free body diagram of forces acting on the rods and the coupler is shown in Fig. 02. Each rod is

subjected to a tensile force P and torsional moment . In the initial stages, it is not possible to find out

torsional moment. Considering only tensile force,

P=A

Where, A is tensile stress area of the treaded portion of the rod and is permissible tensile stress.

The rod is made of steel 30C8 ( = 400Pa) and factor of safety is 5. Therefore,

75KN= A=937.5

From table, ISO metric coarse screw thread of M36 designation is suitable for the rod (stress area =

817 ).For M36 size, the core diameter ( as per table is 31.093 mm.

Trial No.01

d = 36 mm and = 31.093 mm

= =

98.77 Mpa or N /

= 0.098Pd = 0.098( 75KN )( 36mm )

= 264,600 N-mm

= =

44.83 N /

The principal shear stress is given by,

=

=

= 66.69988 N /

And,

= =

= 2.9985

The factor of safety is less than the required value of 5. The next size of ISO metric coarse thread

is M42 with 36.479 mm as minor diameter ( and 4.5 mm pitch. The stress area is

Trial no. 02

d = 42 mm and = 36.479 mm

= =

71.76 Mpa or N /

= 0.098Pd = 0.098( 75KN )( 42mm )

= 308,700 N-mm

= =

32.38754 N /

The principal shear stress is given by,

=

=

= 48.33575862 N /

And,

= =

= 4.1377

The factor of safety is slightly less than the required value of 5. The next size of ISO metric coarse

thread is M48 with as minor diameter ( and 6.5 mm pitch. The stress area is

Trial no. 03

d = 48 mm and =45.802 mm

= =

49.36 Mpa or N /

= 0.098Pd = 0.098( 75KN )( 48mm )

= 352,800 N-mm

= =

21.116N /

The principal shear stress is given by,

=

=

= 32.48 N /

And,

= =

= 6.15745

The factor of safety is satisfactory. Therefore, the nominal diameter and the pitch of threaded portion of the rod should be 48 mm and 6.5 mm pitch respectively. In Fig. 03, the length of threaded

portion of the rod in contact with coupler threads is denoted by l. It is determined by shearing of the

threads at the minor diameter . Equating shear resistance of the threads to the tension in the rod,

= P

Where,

= = = 40 N /

Therefore,

= 75KN

= 13. 569276mm

This length is too small compared with the nominal diameter of 42 mm. In practice, the length l varies from d to 1.25d. Or,

l = d = 48mm

l = 1.25 d =1.25(48) = 60mm

From (i) and (ii) the length of threaded portion l is assume as60mm.

DESIGN OF COUPLER

The two ends of the coupler are called coupler nuts. As shown in Fig. 03, the coupler nuts are integral with the coupler. The outer and inner diameter of the coupler nuts are D and d respectively and length is denoted by l. It acts as a hollow rod. Considering tension,

P =

75KN =

The coupler is made of cast iron and factor of safety is 5.

Therefore,

= = 40 N /

Substitute the above value in Equation.

75KN =

Or, D = 68.49 mm or 70 mm

The standard proportion for D is from 1.25d to 1.5d. Or,

D = 1.25d =1.25 (48 mm) = 60mm

D = 1.5d = 1.5 (48 mm) = 72 mm

From the two result of diameter, it is decided that the dimension for D should be 70 mm.

The coupler nut is subjected to direct tensile stress as well as torsional shear stress due to torque

Check for design

= 352,800 N-mm

r = = = 35mm

J = = = 1,836,023.716

= r = = 6.7254 N /

= 36.785 N /

Coupler is made of cast iron. Cast iron is brittle material. For brittle materials, maximum principal stress theory is applicable. The maximum principal stress is given by,

=

=

= 37.9757 N /

( ) = = = 5.2665

The factor of safety is satisfactory.

Remaining dimensions of the central portion of the coupler, viz. and l are decided by the following

empirical relationships,

d + 10 = 48 mm + 10 = 58 mm

=2 ( 58mm ) = 116 mm

L = 6d =6 (48 mm) = 288mm