Lock Wire Installation
Crosshead Balance Weight Bolt Heads 2802 Delmar Drive, Suite I
Victoria, Texas 77901
Bolt Head “Lock Wire”
On Chicago Pneumatic & Gemini reciprocating compressors, cap screws are typically used in the attachment of reciprocating balance weights to crossheads. These bolt heads have historically been drilled for the installation of “lock wire” after final bolt torquing. To assist the field mechanic with lockwire installation during maintenance efforts, the following installation procedure is provided.
Installation of Lock Wire - Detail Instructions
“Lockwiring” is a technique that uses twisted high strength stainless steel wire to tie cap screw heads together to resist bolts loosening. The wire is laced through the holes such that an attempt to loosen a single bolt tightens the cap screws on each side of it. The cap screw heads are pre-drilled to an appropriate diameter by the fastener manufacturer. If the wire is not installed properly, it is useless for locking purposes.
The wire is twisted after being laced through the holes (a drilled head will usually have two through holes perpendicular to each other to optimize wire position). The wire is usually annealed stainless steel, in order to make it easy to bend and lace.
Cap screws should be lock wired in such a manner that the lock wire shall be putin tension when the part tends to loosen, see Figure 1 below for right hand (RH) threaded fasteners. The lock wire should always beinstalled and twisted so that the loop around the head stays down anddoes not tend to come up over the bolt-head and leave a slack loop. Care should be exercised when installing lock wire to insure that it is tight but not overstressed.
Figure 1: Typical “lock wire” installation - 4 Steps for RH Threaded Cap Screws
Installation Procedure
- Check the cap screws to be lock wired to make sure that they have been correctly torque to the specified value.
CAUTION: Under-torquing or over-torquing to obtain proper alignment of the bolt’s drilled holes is not advisable. The drilled bolt head will usually have two through holes perpendicular to each other to optimize wire position.
- Lockwire bolts in pairs. In adjacent bolt heads, it is desirable that the holes be in approximately the same relationship to each other as shown in Figures 1-1 through 1-4 (for right-handed threads), thus the lock wire will have a tendency to pull the fastener clockwise in a tightening direction.
- Use 0.032” diameter SS wire, (part# 1-PS-157TR in 340 LF roll) made for this purpose.
- Match pairs of bolts so that the holes in the heads are between 2 and 6 o’clock to a line running thru the heads, see Step #1.
- Insert wire through first bolt head, bend around the head and twist ends together in a clockwise direction, see Step #2.
- Continue twisting wire ends together for the paired bolt heads. Insert the top end of the wire into the bolt head’s hole and bend the bottom end around the head. Twist wire ends together in a counter-clockwise direction, see Step #3.
- Cut off excessive length and turn the twisted ends in along the bolt head, leaving no sharp ends to avoid safely hazards, Step #4.
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