Detailed explanation of Ball and Socket Fittings：Locking Pin、Ball eye link、Socket clevis Selection and installation considerations

Ball and socket fittings are the most common type of Special connecting fittings in insulator strings for power lines. They are mainly used to match and connect with ball-and-socket type suspension insulators. Because they directly affect the reliability and safety of the insulator string, they are considered a "critical small component and major risk point" in design selection, on-site installation, and operation & maintenance inspection.

What are ball and socket fittings?

Ball and socket fittings are a type of Special connecting fittings used specifically to connect ball-and-socket type suspension insulators. The typical matching structure is ball head ↔ socket head, and reliable locking is achieved through a locking Pin. Their structure and connection dimensions must match the ball-and-socket structure of the insulator, ensuring that after assembly they can rotate flexibly under load, bear force evenly, do not jam, and do not fall off.

They are commonly used in overhead transmission lines, distribution lines, substations, and groove-type connection scenarios.

Common classifications of ball and socket fittings

Locking Pin

The locking Pin is also called a spring pin. Its mass is only 1/250 of an insulator, but if the insulator string is installed without a locking Pin, or the quality is unqualified, or it is installed improperly, the resulting losses are immeasurable.

Locking Pin types include W-type pins and R-type pins, and they are mostly made of copper material, with good elasticity and corrosion resistance, and are easy to assemble and disassemble.

(a) W pin; (b) R pin

In the structure of groove-type connection, a cylindrical pin is used, and it is locked with either a cotter pin (hot-dip galvanized surface) or a camelback pin (made of copper).

For XP series insulators:

For 160 kN class and below, use W-type push-pull pins

For 210 kN class and above, use R-type push-pull pins

The characteristic of push-pull pins is that when loading/unloading the insulator, you only need to pull the pin out of the pin hole (still hanging in the steel cap socket) and push it in, without removing it. It can be re-driven in, which is convenient for loading/unloading and prevents pin loss.

Diagram showing the positions of W-type or R-type spring pins when locked and unlocked

Closed-end pins used for bolt-tail connecting fittings are mostly made of copper or stainless steel. Their feature is that after the closed-end pin is inserted into the hole, it will automatically spring open, without needing to bend the tail by 45°. It is easier to pull out of the pin hole, works reliably, and is flexible for live-line loading/unloading.

Locking Pin selection quick reference table

Ball eye link

The ball-head connection end matches and connects with the socket head. It is a connecting part shaped like a ball rod, called a ball head. The matching connecting part is the socket head. Fittings with ring-shaped connecting parts at both ends are called hanging rings. According to structural differences, they are divided into Q type, QH type, QP type.

Ball-joint suspension ring structure diagrams:

(a) Type Q ball-joint suspension ring; (b) Type QH ball-joint suspension ring; (c) Type QP ball-joint suspension ring

Technical Parameters of Q-Type and QP-Type Ball Eye Links

Technical Parameters of QH-Type Ball Eye Links

Model designation explanation: Q- Ball head; H- Ring; Number - Nominal breaking load; S- Spacing between plates. Example: Model QH-7 indicates a ball-head hanging ring with a nominal breaking load of at least 70kN; Model QH-21S indicates a sector-shaped elliptical ball-head ring with a nominal breaking load of at least 210kN.

Ball clevis

Ball clevis (Ball Clevis) has many structural forms. The most common is QB type and its extended types. Below is the structural diagram of the QB type ball clevis.

QB type ball-joint suspension plate structure:

(a), (b) Q-×HC type; (c) QB-×XY type; (d) QB-×XH type

Q-×× type ball joint mounting plate:

(a) Parallel ball joint mounting plate with circular holes; (b) Sector-shaped parallel ball joint mounting plate with circular holes

Technical Parameters of Ball Clevis (QB Type)

Model designation explanation: Q- Ball head; B- Mounting plate; Y- Extended type; H- Ring; The number after the hyphen indicates the nominal breaking load. Example: Model QB-7YH indicates a ball-head, ring-type extended mounting plate with a nominal breaking load of not less than 70kN.

In addition to the Ball eye link mentioned above, many fitting manufacturers will design additional Ball eye links for installation needs. When designing and selecting Ball eye links, try to avoid assembly methods where the connection point produces point contact, which causes stress concentration.

Ball eye link with hook or ring

It is a connecting fitting composed of a ball end and a hook, which can be directly hung on the cross-arm pull plate, or connected with a U-bolt on the cross arm.

The advantage of a Ball eye link with an oval ring is that it can be used to connect with a U-bolt with a flange (reinforcement), while a general Ball eye link cannot connect with a flanged U-bolt. Hoisting and disassembly are also more convenient, without needing to remove nuts and closed-end pins. However, to ensure operational safety, the hook should have a locking device. But when a tension insulator string uses this type of hanging ring, the over-pull distance is larger than that of general connecting fittings.

Diagram of ball-head hanging ring structure with hook or loop:

(a) Hanging ring with hook; (b) Ball-head hanging ring with parallel loop hole.

Technical Parameters of Ball Eye Links with Ring (Parallel Ring Hole Type)

Q - Ball head; "-" followed by a number - Nominal breaking load; Additional letters (different from standard DL/T 683-2010, these are manufacturer's markings): M - with hook, H - with eyelet, A - long.
Example: Model Q-7M indicates a ball head suspension clamp with a hook (parallel eyelet type), with a nominal breaking load of not less than 70kN.

Other Ball eye links used together with suspension clamps include U-shaped ball eyes, commonly including non-angle-gap type (QU-×× type) and angle-gap mounting-hole type (QU-×H type). The closed-end pin of this type of product is made of stainless steel, and the rest are hot-dip galvanized steel parts.

U-shaped ball joint structure diagrams:

(a) QU-×× type (without angular clearance); (b), (c) QU-×H type (with angular clearance mounting holes)

Socket clevis

Socket clevis is composed of a socket head and a single/double clevis plate, and it is a typical Special connecting fittings that directly mates with the socket head end of the insulator steel cap.

W type is a single-link Socket clevis

WS type is a double-link Socket clevis

Drum-type W Socket clevis is used to improve bolt bending and wear at the groove connection (common derivatives such as W, WS, WSH)

The single-link Socket clevis (W type) is suitable for insulator model XP-70, and can be equipped with arcing horns. Socket clevis is only used at positions where insulators are directly connected, and cannot be used for other purposes.

ZHX hanging rings

ZHX hanging rings are mostly used together with suspension clamps, for overhead transmission lines, distribution lines, and substations to fix conductors and ground wires on poles and towers.

The closed-end pin is made of stainless steel, and the rest are hot-dip galvanized steel parts. Common models include ZHX-15B, ZHX-20B, ZHX-25B, ZHX-25M.

ZHX hanging ring structure diagram

Rod end ball joint

Rod end ball joint is double-ball-head, used for connection between two sockets (insulator steel cap sockets), such as the mutual connection between two rod insulators, generally used in high-voltage test halls.

QS type ball joint and connecting rod structure dimensions and assembly diagram:

(a) Structural dimensions diagram; (b) Assembly diagram 

Installation and common mistakes

Standard installation steps

Alignment

Ensure the insulator string hangs naturally and the connection points are coaxially aligned.

Forcibly inserting the ball head without alignment is prohibited, because it can easily cause deformation of the socket opening, local crushing of the ball head, etc.

Insert ball head / socket head

After the ball head is inserted into the socket head, it should reach the designed positioning depth. If "jamming or half-insertion" occurs, first check for burrs, foreign objects, and misalignment. Do not knock or smash violently.

Install locking Pin

Different pins have different acceptance standards:

Push-pull pin (W type / R type) acceptance standard

After the pin body passes through the hole position, it must fully enter the locking position

After the push-pull action is completed, the pin should be reliably retained, without half-insertion or rebound withdrawal

Split pin acceptance standard

After insertion, the split legs must be fully opened, and locking must be reliable

A common mistake is that the split legs are not opened, making it look installed, but actually not locked

Closed-end pin acceptance standard

After insertion, it should automatically spring open and lock, without secondary bending

At the same time, confirm that the locking end is fully opened to prevent false locking

Recheck

Rotation check: the connection point should rotate flexibly at a small angle, and should not be stiff or seized

Seating check: the end position of the locking Pin is correct and not loose

Interference check: no interference with clevis plates, clamps, grading rings, arcing fittings, sag fittings, etc.

Avoid point contact causing stress concentration

At the ball-and-socket connection, if the assembly method causes point contact at the connection point, it will lead to:

Local stress concentration

Higher risk of wear, deformation, and cracks after long-term operation

In extreme cases, connection failure may occur

It is recommended to select surface contact / reasonable contact methods as much as possible, and avoid connection combinations with "poor overlap and eccentric wear loading".

Common mistakes summary table