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Self-locking of Joint Connections

Release time:2019-11-04 From: Zhengnai Industry in China

Cylindrical nipple connection is more self-locking

The self – locking property of cylindrical nipple is better than conical nipple. We suggest you choose the cylindrical connector first.
 

Self-locking force definition of nipple

The self-locking force of the nipple is the friction force between the threads that restricts loosening after the nipple is screwed into the nipple hole of the electrode and a certain tightening torque is applied.
 

3 factors affecting self-locking force

The degree of self-locking force is affected by static friction Angle, dynamic friction Angle of graphite electrode and rising Angle of nipple thread.


 

3 ways to enhance self-locking force

  • The shape and pitch of connection thread, triangle thread and conical thread have large contact surface, which is conducive to increase the friction between friction and achieve a close connection, and reducing the pitch can play the same function.
  • Machining accuracy, improve machining surface smoothness and reduce mating gap of male and female threads are helpful to increase the self-locking force of connection.
  • Increase the friction between the contact surfaces of the thread to achieve self-locking.

Take appropriate anti-loosening measures.

Apply tightening torque to reduce resistance
Tightening torque is the torque required when the connector is screwed into the electrode connector hole to achieve a close connection. The contact resistance at the connection between graphite parts and graphite parts with smooth surface varies with the applied pressure, while the contact resistance decreases when the pressure increases. In order to minimize the contact resistance at the contact position when the electrodes are connected, in addition to the two important conditions of electrode material and machining accuracy, which must meet the technical standards, a certain tightening torque must be applied when the electrodes are connected in the steel works.

Specifications of RP/HP graphite electrode


Item Unit RP HP
300-500mm 550-800mm 300-500mm 550-700mm
Electric resistivity Electrode μΩm 7.5-8.6 7.5-8.8 5.5-6.8 5.5-7.0
Nipple 4.5-6.0 4.0-5.5
Bending strength Electrode Mpa ≥7.5 ≥6.5 ≥10.0 ≥8.5
Nipple ≥16.0 ≥18.0
Elastic modulus Electrode Gpa ≤9.0 ≤13.0
Nipple ≤13.0 ≤15.0
Bulk density Electrode g/cm3 1.54-1.62 1.62-1.70
Nipple 1.70-1.78 1.72-1.80
CTE(100-600℃) Electrode 10-6/℃ 2.20-2.60 1.80-2.20
Nipple 1.50-1.90 1.50-1.80
Ash % ≤0.3 ≤0.3

 

There are 2 reasons why the tightening torque is less than the specified

The electrode processing quality is not good, such as rough working surface or poor clearance of thread
The steel mill does not tighten the electrode and the nipple when operating the nipple, that is, the required tightening torque is not achieved.
 
 

3 measures to prevent loosening

To prevent the nipple from being connected to the electrode and causing looseness, some auxiliary measures can be taken.
  • 1. Use a connector plug when connecting.
  • 2. Use the connector pin.
  • 3. Use a nipple paste.

 

Two ways to use the nipple

Connector plugs are a common method of preventing loose connections.
  • 1.There are four or six shallow holes in the appropriate part of the finished nipple, The nipple plug is pressed into the shallow hole, and the nipple plug is mixed by a certain ratio of graphite powder, resin, curing agent and coal pitch, and then pressed into a cylinder.
  • 2. The method of filling a small amount of liquid hard asphalt into the drilled shallow hole is adopted, but this method is performed before the male thread is processed on the nipple surface, so that the male thread is simultaneously processed on the cooled solid hard asphalt surface.
 

Connector size

The connector plugs are generally 20 to 30 mm in diameter and 10 to 20 mm in thickness.
 

Shortcomings of nipples

The connector plug securely bonds the electrode to the connector to effectively prevent the connector from loosening. The disadvantage is that it is difficult to remove the stub without damaging the connector hole once the connector breaks.
 

 

How to use the nipple pin

The nipple pin is turned into a strip by graphite rod. When the electrode is connected, the hole is drilled at the nipple, and then the corresponding size of the nipple pin is inserted. The nipple pin can fix the electrode and the nipple. When necessary, pull out the nipple pin. Separating the electrode from the nipple, but drilling holes in the steelmaking furnace and placing the nipple pin is also troublesome.
 

How to use the nipple paste

The nipple paste is the easiest way to prevent the nipple from loosening. It is formulated with a carbon paste that can function as a bonding function. After carbonization, the electrode and the nipple can also be bonded together. When connecting the electrodes, put a small amount of carbon paste directly into the electrode nipple hole and screw the nipple. The binder is soft asphalt. The disadvantage is that it is difficult to remove the stub without breaking the nipple hole when the nipple breaks.
 
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