Turnbuckle: Round Rod Adjustable Screwed Joint
One of the rods in this adjustable screwed joint should have a right-hand thread, and the other rod should have a left-hand thread. The rods are attached to a couple with a threaded hole. This couple can be either closed or open. The open-body turnbuckle is depicted in the preceding figure. The closed turnbuckle is explained further below. We will create a closed-body adjustable screwed joint in this article. The closed-body turnbuckle has a unique design, featuring a hexagonal or rectangular shape in the middle and on both ends. This design allows for the tightening or loosening of the rods using a spanner when needed.
A round iron rod can be used instead of a spanner to tighten the turnbuckle.
The iron rod is inserted into a hole in the coupler, as shown dotted in the schematic diagram of a hexagonal turnbuckle below.
The turnbuckle is a type of rigging screw frequently employed in engineering applications, such as in airplanes, to tighten cables and ropes. To reduce weight, the above-shown turnbuckle is hollow in the middle. The two ends of the rods can also be seen in this case. The rods and turnbuckles do not require to be made of the same or different materials. It is determined by the force acting on the joint.
Turnbuckle / Rigging Screw / Adjustable Screwed Joint Design
The device pictured is a Turnbuckle, which is also referred to as a Rigging Screw or an Adjustable Screwed Joint. When designing it, it is necessary to consider that the turnbuckle will experience an axial load P, as indicated in the diagram below. As a consequence of this load, the threaded rod will be under tensile
stress and the amount of stress can be calculated using the following
equation:
t = PA = P4(dc)2
Turnbuckle: An adjustable screwed joint for round rods, where
dc = the threaded rod’s diameter.
The torque required to drive the rods is given by
T = P tan(α+φ)(dp/2)
Where,,
α = Helix angle,
tan φ = Friction coefficient between threaded rod and coupler nut
dp = Threaded rod pitch diameter or mean diameter
The torque-generated shear stress,
= TJdp2= P tan(+)dp232(dp)4dp2 = Ptan (+) 8 (dp)2
= 8P(dp)2 tan + tan 1-tan tan
Turnbuckle: Round Rod Adjustable Screwed Joint
The standard values for tan α, tan φ, and DP are as follows: tan α = 0.03, tan φ =
0.2, and dp = 1.08 dc
By substituting these values into the preceding expression, we obtain
= 8P(1.08dc)2 0.03 +0.21-0.030.2 = 8P4 (dc)2 = P2A = 12
….A=4 (dc)2
Because the threaded rod is subjected to both tensile and shear stress, the
maximum principal stress is
t (max) =12 + 12 (t)2+42 = 12 +12(t)2 + (t)2
= 0.5t + 0.707t = 1.207 P/A
Given a margin for the more significant coefficient of friction, the maximum principal
stress is 1.3 times the everyday stress.
As a result, when designing a threaded section, we will use 1.3 times the average load as the design load.
As a result, the Design load (Pd) = 1.3 P.
The following steps can be taken when designing a turnbuckle:
1. The rods’rods’eter
The diameter of the rods (d) can be calculated by considering the rod’s roots’ tearings.
We know that the tearing resistance of the rod’srod’sads = 4 dc2 t
Turnbuckle: Round Rod Adjustable Screwed Joint
Let us equate the design load (Pd) with thread tearing resistance, and we can write.
Pd = 4 dc2 t
Turnbuckle: Adjustable Screwed Joint for Round Rods, where dc = Core diameter of the rod’srod’sads and
σt = Permissible tensile stress for the rod’srod’srial.
The core diameter of the threads is calculated using the above expression.
The nominal thread diameter or rod diameter, corresponding to the core diameter, can be obtained from the table below, assuming coarse threads.
2. The coupler nut’snut’sth
The length of the coupler nut (l) can be calculated by considering the shearing of the threads at their roots in the coupler nut.
We know that the shearing resistance of the coupler nut threads = (π dc × l) τ Where τ = Shear stress for the coupler nut material.
When the design load is equal to the shearing resistance of the threads in the coupler nut,
Pd = (π dc × l ) τ
The value of l is calculated using this expression.
In practice, the length of the coupler nut (l) is between d and 1.25 d for steel nuts and 1.5 d to 2 d for cast iron and softer material nuts.
The length of the coupler nut should also be checked for thread crushing. We know that the threads in the coupler nut have a crushing resistance.
= 4(d)2-(dc)2 nlc
Turnbuckle: An adjustable screwed joint for round rods,
where
σc represents the crushing stress induced in the coupler nut, and n represents the number of threads per mm length.
We have calculated the design load by relating it to the crushing resistance of the threads.
Pd = 4(d)2-(dc)2 nlc
Turnbuckle: Round Rod Adjustable Screwed Joint
The induced crushing stress (σc) can be calculated using this expression.
3. Coupler nut outside diameter
The external diameter of the coupler nut (D) can be calculated by considering the tearing at the coupler nut.
We are aware of the Tearing resistance at the coupler nut.
=4 D2-d2 t
Turnbuckle: Adjustable Screwed Joint for Round Rods, where σt = Permissible tensile stress for the coupler nut material.
We can write by equating the axial load with tearing resistance at the coupler nut.
P=4 D2-d2 t
Turnbuckle: Round Rod Adjustable Screwed Joint
The value of D is calculated using this expression.
In practice, the diameter of the coupler nut (D) used from 1.25 d to 1.5 d
4. The couplcoupler’side diameter
The outside diameter of the coupler (D2) can be calculated by considering the couplcoupler’saing.
We know that the couplcoupler’saing resistance is given by
=4(D2)2- (D1)2t
Turnbuckle: An adjustable screwed joint for round rods, where D1 is the couplcoupler’ssde diameter. It is commonly taken as (d + 6 mm) σt = Permissible tensile stress for the coupler material.
We can calculate the axial load by relating it to the couplcoupler’saing resistance.
P =4(D2)2- (D1)2t
This expression can calculate the value of D2. In practice, the outside diameter of the coupler (D2) is assumed to be 1.5 d to 1.7 d.
If the coupler section is to be hexagonal or rectangular to accommodate the spanner, it may be circumscribed over the circle of outside diameter D2.
