Joining armoured cables and avoiding bearing breaks

Joining armoured cables and avoiding bearing breaks

By Grant Laidlaw

We answer two questions in this edition: how to join electrical armoured cables and how to avoid (and fix) bearing breaks on fans and pumps.

 Riaan asks: Hi Grant. We want to join some electrical armoured cables, but how? What do you suggest? Thanks. 

Hi Riaan, I think I can assist you here.

Although it is never ideal to join electrical cables, there are times when you need to do so. Jointing systems are applicable up to any practical size of cable and may be followed when it becomes unavoidable to join a pair of cables.

Let us begin by looking at what the South African National Standards (SANS) have to say in SANS 0142.

Clause 7.20.10.1 reads: “Joints and terminations of cables, cores and conductors shall be made in accordance with manufacturers’ instructions. Flexible cables or conductors shall only be joined using acceptable cable couplers or manufacturers jointing kits.”

Epoxy sealed jointEpoxy sealed joint.

Cable joints

An epoxy sealed permanent jointing system may be used for all cables, up to practically any commercial size and voltage. This type of joint allows you to make a join that is encapsulated in a resin.

These finished cable joints may be buried in a trench and they are, for all intents and purposes, as good as the cable itself.

First, the cores are joined with crimping or solder ferrules (see instructions in the specific kit). Each is wrapped with a specially supplied insulating tape and a transparent mould, which has had the stepped ends cut back for this to neatly fit over the cable. Pouring funnels are then fitted and a resin is poured into the mould. When the resin has set, the funnels are discarded and the joint is ready for use. In each case, follow the instructions that are furnished with the specific kit.

Typical steps are:

  • Select the correct size cable joint for your cable.
  • Cut back the stepped ends of the mould to leave a diameter that neatly fits over the cable jacket.
  • Lay out the cables in what will be their final installed position. Cut the ends so that they just touch.
  • Mark the end position of the mould onto the outer insulation/jacket of the cables.
  • Mark 25mm in from each marked point. Strip back the outer insulation/jacket from these two points.
  • Leave the copper conductors that lie intact among the armouring (if present). If no copper strands are found, leave some of the galvanised strands intact. Push them out of the way for now. Cut back the galvanised steel strands of the armouring layer.
  • Mark in a further 10mm from where the conductors have been stripped. Remove the cable bedding from these points inwards.
  • Strip insulation from the conductors. Strip each by half the length of the ferrule plus a further 10mm.
  • Connect the matching cores and then the armouring, using the ferrules supplied in the kit.
  • Insulate each ferrule individually with two laps of applicable insulation tape, stretched tightly.
  • Clean those parts of the cable jacket that will go inside the mould, using the aluminium oxide cloth supplied in the kit.
  • Double-check that the ferrules have been properly crimped with the correct crimping tool. You do not get a second chance!
  • Make an O-ring out of the No. 23 tape on the cable jacket at each side. This will help form a seal for the resin when it is first poured.
  • Clip the mould over the cable. Fit the pouring funnels to the mould. Bind the ends of the mould with tape, to complete the formation of the seal. The resin and hardener go into the pouring bag.
  • Mix the pour resin according to the instructions (contained in the pouring bag) and fill the mould with this.
  • When the resin has hardened, saw off and discard the pouring funnels. The jointed cable is now ready for use.

Riaan, that’s really all there is to it.

Joints and terminations of cables, cores, and conductors shall be made in accordance with manufacturers’ instructions.

Thomas asks: Grant, we work on bigger air-conditioning systems and often seem to have bearing breaks on fans and pumps. Why does this happen? Also, what to do? Thanks.

Hi Thomas, let us start by examining the failed bearing following a step-by-step approach.

The examination of a failed bearing is a valuable source of information to identify its installation and operating conditions; therefore, it must be carried out methodically and with precision.

Before removal, note the following:

  • Noises
  • Vibrations
  • Rise in temperature / loss of lubricant
  • Contamination.

During removal

  • Remove the end caps, seals (do not wash them), and grease, then place them in a clean place for later examination.
  • Record the torque of the tightening nut that clamps the ring faces.
  • Note the axial and radial positions of the bearing (identification marks on rings with respect to the shaft and the housing) and the direction of installation.
  • Check the fit in at least two planes (shaft and housing).
  • Note the condition of the seats and the surrounding parts.

After removal

  • Perform visual examination
  • Dismantle the bearing
  • Examine the components
  • Analyse the grease, and check for foreign particles by washing and filtering.

Appearance of failures

  • Fatigue spalling, cracking, and removal of material fragments.
  • Surface spalling.
  • Stains resulting from flaking of surface material / seizing. This looks like matte areas with the removal of material, brown marks from heating, deformation of rolling elements, local melting, and scoring of the metal.
  • Indentations caused by deformation: Ball or roller indentations corresponding to the space between the elements. The bottom of each indentation is shiny, but the original grinding marks are still visible. The material has been displaced without wear.
  • Raceway indentations due to abrasive wear: Indentations that may or may not correspond to the spacing of the rolling elements. Removal of material due to the vibration to which the bearing is subjected when stationary.
  • Wear.
  • General wear of the rolling elements, raceways, and cage. Grey tint (due to the effect of abrasive contamination).
  • Pitting and fluting.
  • Pits with sharp edges or sequence of narrow parallel grooves, resulting from the leakage of an electric current.
  • Nicks, cracks, fractures.
  • Impact load dents, removal of surface material, cracks, fracture of rings.
  • Fretting corrosion: Red or black discolouration of the bearing contact surfaces, on the face, in the bore, or on the outside diameter.
  • Corrosion: Local or total oxidation of the bearing surfaces, both inside and outside.
  • Discolouration: Discolouration of the raceways and rolling elements: overheating of protection oil.
  • Cage failure: Bending, wear, fractures.

Causes of bearing failure

The causes of failure can be related to four main sources:

1. Poor fitting procedure

  • Inadequate or improper fitting equipment and method.
  • Contamination during fitting.
  • Excessive force.
  • Poor construction of bearing seats: shafts and housings out of tolerances, poor lubricant access, misalignment.

2. Operating conditions

  • Overloads, accidental or otherwise.
  • Vibration in service or when stationary.
  • Excessive speed.
  • Shaft bending.

3. Environmental conditions

  • Ambient temperature too low or too high.
  • Conduction current across the bearing.
  • Contamination by water, dust, chemical products, textile debris, and so on.

4. Lubrication (deterioration can result from one or more causes)

  • Incorrect choice of lubricant.
  • Unsuitable quality.
  • Inadequate re-lubrication frequency.

Thomas, I hope this helps with your issue around bearing failure.


Thank you for all your questions. Send your problems (and sometimes your creative solutions) to acra@netactive.co.za with “Solutions Page” in the subject line. You may include pictures.


References:

  • ACRA
  • South African National Standards
  • 3M

 Click here to read the June 2018 issue of RACA Journal


 


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