What is a Bolt type tension clamp?
A Bolt type tension clamp is a kind of power line fitting used in overhead transmission/distribution lines. Its function is to firmly anchor the conductor or the ground wire/shield wire on the tension insulator string or the tower/pole structure, so as to bear all or most of the mechanical tension of the conductor at that location. The clamping method uses the vertical pressure of U-bolts, which causes the pressure block and the groove of the clamp to generate friction on the conductor to fix the conductor. Its physical photo and structural diagram are shown in Figure.
Figure:Physical photo and structural diagram of Bolt type tension clamp
In anchoring fittings, a Bolt type tension clamp is often described with closely related names depending on the application and structure, such as a bolted type tension clamp, a bolted dead end clamp, or a dead end strain clamp. In some design families (especially where the body shape is characteristic), it may also be referred to as a quadrant strain clamp.
Working principle of a Bolt type tension clamp
Bolt preload force → clamping force (normal force) → friction / biting effect → transfer the conductor tension to the tower material / insulator string. The bolt tightening torque must be applied according to the specified torque, symmetrically and evenly in steps, to prevent "insufficient clamping leading to slippage" or "over-tightening causing strand damage".
Types of Bolt type tension clamp
According to manufacturing materials, Bolt type tension clamps can be divided into malleable cast iron Bolt type tension clamps, stamped Bolt type tension clamps, and aluminum alloy Bolt type tension clamps. These clamps can be removed after installation and can continue to be installed elsewhere while still having the same function and effect. Therefore, they are called live clamps, also called the first type of clamp.
Malleable cast iron Bolt type tension clamp
The clamp body and pressure plate are malleable cast iron parts, the closed-end pin is a stainless-steel part, and the rest are steel parts. The malleable cast iron and steel parts are hot-dip galvanized. It is suitable for installing small and medium cross-section aluminum stranded conductors and ACSR (aluminum conductor steel reinforced).
Stamped Bolt type tension clamp
The stamped Bolt type tension clamp is formed by steel plate stamping. The closed-end pin is stainless steel, and the rest are steel parts treated by hot-dip galvanizing. This type of tension clamp belongs to magnetic materials, and its electromagnetic loss is relatively large, so it has gradually been replaced by aluminum alloy Bolt type tension clamps.
Aluminum alloy Bolt type tension clamp
A Bolt type tension clamp cast from aluminum alloy material is called an aluminum alloy Bolt type tension clamp. It uses high-strength aluminum alloy casting, with high strength and good corrosion resistance, and has an energy-saving effect. It is mainly used for tension anchoring installation in transmission lines, distribution lines, and substation equipment. In many practical purchasing and specification documents, the aluminum alloy version is also listed as an aluminum alloy strain clamp.
The structure of the Bolt type tension clamp is shown in Figure 3-5.
(Figure 3-5 Structural diagram of Bolt type tension clamp: (a) NLD-×× type; (b) ND-×× type; (c) NLL-×× type)
Inverted installation (for operating lines)
If, for an operating line, the tension clamp needs to be installed in the manner shown in Figure, then the clamp is called an inverted Bolt type tension clamp. This installation method is called "inverted installation." That is, on the tension side (span side), there is no U-bolt fixation, and all U-bolts are installed on the jumper side. In other words, the clamp must not be installed in reverse; otherwise, the mechanical strength of the clamp will be reduced, and it may even cause a fracture accident.
Other types of Bolt type tension clamp
In addition to the above Bolt type tension clamps, according to the needs of transmission and distribution lines, power fitting research units and fitting manufacturers have designed and manufactured the NLW type Bolt type tension clamp shown in Figure.
For example, the NLW-1 type Bolt type tension clamp is applicable to stranded conductor diameter ∅4–14 mm, its failure load is not less than 64 kN, and the product mass is about 1.8 kg. Another example is the NLW-2 type Bolt type tension clamp, applicable to stranded conductor diameter ∅6–17 mm, its failure load is not less than 70 kN, and the product mass is about 2.32 kg.
Main applications
Used at tension towers, angle towers, terminal towers, and other positions requiring "fixed without slippage";
Transfers conductor tension from the line to the insulator string / fitting string / tower structure, providing anchoring and load-bearing functions.
(In the category of anchoring hardware, these applications are also grouped under dead end strain clamp categories, especially for termination and anchoring points.)
Typical features
Easy to install and can be removed and reused (compared with compression "dead-end clamps," the bolt type is detachable and convenient);
Suitable for a certain range of conductor cross-sections (different models correspond to different conductor diameters/cross-sections).
Simple analysis of tension clamp failure in operation
Tension clamps include bolt type, compression type, etc. The failure of tension clamps in operation is mainly manifested in the following aspects:
(1) Fatigue failure caused by too small curvature radius at the outlet of the bent part of the Bolt type tension clamp, resulting in excessive bending stress.
(2) For compression type tension clamps, if the crimping quality does not meet requirements or the crimping performance deteriorates during operation, oxidation occurs at the contact surface, the contact resistance increases, and local heating occurs.
(2) There are gaps inside the body of the compression type tension clamp, and long-term exposure to moisture leads to corrosion.
(3) The tension clamp and the conductor are two different metal materials, and bimetallic corrosion occurs at the connection.
(5) Excessive stress damage occurs in the contact area between the tension clamp and the conductor due to micro-wind vibration, conductor galloping, or other factors; and vibration causes the grip strength of the steel anchor to decrease, resulting in the conductor slipping out of the clamp and falling off.
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