Maintenance Detective

A messy powder filler is fed by a pair of pneumatic indexing cylinders. They gate bottles into the filler one at a time. But there is a problem: When you start the line it runs perfectly. Crisp, fast index strokes. About two minutes in, the cylinders start getting lazy. Strokes slow down. Bottles start arriving late. Then the indexers barely move and the filler starves. Stop the line for a bit. Restart. Perfect again... for about two minutes. Over and over. All shift. You hav

First time it happened, no big deal. We just power cycled the machine and it went away. One drive in a bank of them dropped out mid-shift: the fault said something about "F111 Safety Hardware". Machine just shut down and wouldn't run. No E-stop pressed. No gate opened. Safety relay had all green lights, no issue there. After power cycling, the machine fired up and ran the rest of the shift with zero complaints. Then nothing for three weeks. Then it came back. Once a week

You're called to a robotic palletizing cell that hard e-stops every time a finished pallet exits through the light curtain at the discharge. It won't reset until they remove the pallet, the operator is manually pulling pallets out of the discharge every pallet. He says he's never seen it do this before. What should I check? Answer: The light curtain is doing its job correctly. A light curtain doesn't know the difference between a pallet and a person, and it's not supposed to

While working on a machine, you touch a control transformer and it's blisteringly hot. Well over 200 degrees. You check it and it's not carrying much load at all. It's cycling about every 8 seconds, only on for maybe 2 seconds... and pulling less than half the rated load. It's switching on the primary side with a SSR. Why is it running so hot? What could we do about it? Answer: Every time the SSR closes, it's not just turning on a small downstream load, it is re-energizin

Let's play a game for all the maintenance players out there. You come in one morning and there is a note in the passdown from night shift: 6 hours of downtime, they changed 4 components: control transformer, a fuse, a contactor and a relay. There are many possibilities here... but what assertions could we make? What is the most likely explanation? And what do you do about it? (There is no right answer here, but there's a single scenario I've seen produce this exact outcome ab

Let's play a game for all the maintenance players out there. You've got a conveyor. The overload trips. Standard stuff - you reset it and put it back in service. Except now the conveyor is running backwards. What's the most likely cause? Answer: The backwards rotation and the overload trip are both signatures of single phasing. That means one of your wires is not carrying continuity to the motor. When you lose one leg of your three-phase supply, the motor loses its phase seq

Let's play a game for all the maintenance players! You get a call on a 240V fan motor that keeps throwing its overload. You do your homework. You pull out your meter and check the resistance across all three winding sets - balanced. You check winding to ground - no short. The motor runs. It just trips. Over and over. So what's going on? First: check your mechanical load before you go electrical. Spin the fan by hand with the motor de-energized. It should turn freely with min

Lets play a game for all the maintenance players! Suppose you're called to a case sealer that's gluing and closing the flaps on a corrugated case. You arrive and 1 box in four has flaps that are opening as it comes out of compression. Every open flap you look at has glue on both contact points of the open flap. What are the next things you should check? What common causes are there? Now here's where most techs get it wrong: Start with case dimensions coming out of the former.

Suppose you have a safety circuit on a machine that won't reset, no matter what you do. After checking all the doors, you go open the cabinet and see that the safety relay is showing "channel 1 light is on" and "channel 2 light" is off. What happened, what am I looking for, how do I need to proceed? Answer: One channel light on and one channel light off is an input fault on a safety relay. The relay is detecting a fault because the two channels didn't make simultaneously. D

Suppose you have a conveyor shut down while you're trying to run and you open the cabinet and see "DC Bus Overvoltage" fault on a VFD. What could you do about that? What could be the cause? Answer: Something is causing that motor to overrun the speed the VFD is driving it. When the lug inside the motor spins faster than the magnetic field, it increases voltage to the DC Bus and you get this relatively common fault. Most of the time, the cause is one of these 2 things: Motor

Suppose you have an industrial machine that's stopping with no indication of why and no fault. Power is consistent, it just stops now and then. What is the most probable single cause? Answer: It's probably a bad contact (loose wire or in a relay) in the run circuit. The run circuit of a machine is typically a "latching circuit" that indicates to the machine that the operator is telling the machine to run when it's "high" and that the operator wants the machine to stop when

You have a standard DC brush motor on a DC speed control driving a production conveyor. One day, the operator tells you, "The motor is running at full speed and I can't turn it down." What's the most likely single cause and how could it have been prevented? Drop your answer in the comments before scrolling. Answer: Brush dust buildup shorting the negative brush to ground. Carbon brush dust is conductive. As it accumulates on a brush grommet, it can create a short to ground.

Suppose you look at weekly passdowns, and one of your plastic articulated conveyors has broken the chain multiple times in the last week. You have lots of these conveyors, but this one is breaking and getting repaired repeatedly. They're all about the same age. This one isn't all that different from the others. It does start and stop under load, but they all do. You go look at it and everything you can see while it's running seems fine. No jumping or bad mechanical components