Relay Failure

Hi All,

I had a cross tie relay (it is the same unit as the master relay provided with the kit) go bad on the plane - what happened is it would click but not close the circuit .

The relay was mounted sideways for convenience with the connections; and, I don't know if the mounting position was a factor since the core is moving horizontally in this case. Since the cross tie is an emergency function only in the event one of the plane's two electrical systems dying - it certainly does not get the usage the master relay does.

Sometimes an electrical component will just plain die; but I am wondering if my choice of mounting position may be partly to blame.:confused:

Anyone have any experience with relays dying or experience with other than vertical mounting positions for them?

Thanks,

Dave S
KF7 Trigear
912ULS Warp

Hi Dave,

Generally, these type solenoid relays are not designed to be mounted sideways. Internal parts can gall & bind. They work best when mounted with the plunger vertical. Master solenoids (or your cross tie unit) should be mounted with the domed housing up, so that the coil pulls the plunger (armature) down when activated. Thus gravity helps to keep it engaged, & bouncing over rough ground would not cause it to chatter or momentarily break contact. Also, high "G" maneuvers in flight would help to keep it engaged.

If you have a starter solenoid (like with a Lycoming), it should be mounted just the opposite ("upside down") because high "G" maneuvers or jolts could possibly activate it if mounted the other way around - not a good thing for your starter or ring gear.

It sounds like the coil is probably ok, so your unit may indeed be "stuck" or binding.

It's also good to keep in mind that a diode is a good idea to help prevent arcing & damage to the start or master switch that is activating the solenoid. I'm sure that this has been covered before on this forum. I can detail the specifics of that if anyone needs a refresher.

Mr. Evens,

It's always fun to learn something new.

This was an interesting statement. I’ve never seen an orientation arrow on a contactor so I did a bit of research. We’ll call it a shameless rip-off of Mythbusters.

I checked some aircraft to see if the mounting is always oriented to prevent inadvertent contact or opening of the points on high current contactors. Turns out to be true... some of the time.

I grabbed another airport bum and we looked at his Cessna 177RG with a Lycoming engine (his cowling was off and mine is sealed up,) and a Cessna 152 with a Lycoming.

On the 177 the starter solenoid was facing down and battery contactor was up. That’s in keeping with the suggestion. However, the external power contactor is mounted sideways.

The Cessna 152 had both the battery and the starter contactor mounted up. But note the 152 uses contactors with terminal lugs on the top and not on the side.

So just to be sure what’s going on inside, we grabbed a couple of old contactors and opened them up.

Here’s what we found

If you have the crimped on dome type contactor with terminals on the side the solenoid core moves down when energized. The internal spring holding the contacts open is fairly weak and it could possibly close with a strong thump on landing.

If you have the terminals on the top, the solenoid core moves up when energized. If mounted upside down it could make inadvertent contact with a hard landing. This type needs to be mounted up for the starter. That is in keeping with how they were mounted on the airplane.

So what about the battery contactor? The 177 battery contactor was mounted up so a hard landing would keep the contacts closed. However, the 152 top terminal battery contactor was also mounted up which could possibly open in a hard landing.

Does that bust the theory? Not at all.

What I don’t know is how many G’s it takes to knock open the solenoid contacts when it is energized. The magnetic field on the contactor is very strong. I hooked one up to a power supply and a lamp and slapped it in my hands very hard to see if it would momentarily open. If it opened, the lamp would blink. Not very scientific but certainly more than a few G’s. I was unable to get it to open.

Lastly is the galling issue. The top terminal type has the plastic coated iron core sliding in a smooth phenolic sleeve. It would be near impossible for it to gall when mounted in any position. The side terminal type contactor has a polished metal core with about 1/8 inch clearance sliding in a polished sleeve. It too would be near impossible to cause any galling.

In the end, I would say mounting a contactor inverted for a starter solenoid is probably a good practice and may save you a starter and ring gear. Orientation it is something to keep in mind if you have a bus you don't want powered intermittently should you hit some extreme turbulence or bang a landing.

On the other hand, I think the orientation for a battery contactor is probably irrelevant. I don't think Waldo Pepper could pull enough G's to open the contacts in flight. If you hit hard enough to open the contacts on an energized contactor when landing, the airplane is not likely to be reusable. Mounting sideways should present no problem, either. The solenoid moves once at the beginning of the flight and once at the end of the flight. It's not likely to wear out faster in sideways vs. vertical.

Attached are pictures of the units we looked at for the education.

The first picture is a domed side terminal type. Note the wimpy spring that holds the contacts open. The core moves down to close the contacts.

The second picture is a top terminal type. Note the core must move up to close the contacts.

Third picture is a factory installed external power contactor mounted sideways. It is 40 years old with 6000 hours and has never failed.

One interesting side benefit of opening some contactors was to see the quality. The top terminal type looks far more robust on the outside. However, the contacts on the inside are small in comparison to the large terminal bars and contactor ring of the domed side terminal type. The domed side terminal type is definitely the superior of the two.

Thanks again, John, for the enlightenment!

John Pitkin
Greenville, TX

Thank you, Mr. Pitkin. It seems as though the group can always rely on a thorough analysis of a question or problem from you. I must agree with your opinion that the master solenoid orientation has probably been over-analyzed and is not really critical. I believe that these guidelines have been passed on and accepted by many of us for generations. I know that I first learned of this orientation issue with regard to these Series 70 White Rodgers solenoids (may have been built by Allen-Bradley back then?) almost 40 years ago when I first started building & flying "real" airplanes. Here is a link to a White Rodgers site that specifies the recommended mounting position as vertical, with the dome or cap down. http://www.emersonclimate.com/Docume.../R-4003web.pdf

There are other solenoid/relays that are designed for horizontal mounting.

While it may appear that the possibility of the plunger sticking while in the horizontal position is slim, I have heard of it happening - again, maybe an old-wives tale? There is definitely less chance of excessive wear or binding with vertical positioning. Maybe there was softer, less wear resistant material used on older units?

I guess I've probably beaten the subject to death. It would be interesting to see what Dave's unit looks like when taken apart. Let us know, Dave.

John E & John P,

I want to thank both of you for your input. This is valueable and I have learned from both of you.

I don't have the solenoid disassembled yet, but will do that as soon as I have time.

The failed unit is a series 70 R-W.

I will heed the manufacturer's recommendation on mounting position - Generally, a manufacturer has a reason for a recommendation based on their knowledge of the product - this one I was not aware of when the unit was originally installed.

Thanks again and I'll get back once I get the can opened/inspected and have a little better knowledge of what happened inside this one. Hoping I find something definitive I can take a photo of.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp Drive

Good Morning,

Today I disassembled the offending relay to see what exactly was going on inside.

Photo 1 shows the internal parts to be in like new condition with regard to wear and tear. It was pretty clear what was not a problem. No dirt, no foreign material, no corrosion, no wear, no galling, no pitting, no burned contacts.

Inspection and measurements of the moving parts turned up the following points: 1) The radial play (side to side) of the core was 0.140" according to my dial gauge 2)There is a certain amount of slop between the copper contactor washer and the core/pin it is mounted on to pull it down. 3)The pull is limited by an anvil at the bottom so the difference between what the pull needs to be and what the limit is, is very little - when it hits the bottom - it doesn't pull down any further - period.

Photo 2 shows what happens if the core is manually pushed down to the limit and to the one side - meter shows no contact. (note the core moves further to the edge in this photo compared to photo 3)

Photo 3 shows what happens if the core is manually pushed down to the limit and to the other side - again - the meter shows no contact. (note the core does not move as close to the edge of the case in this photo but still no contact)

Photo 4 shows what happens when the core is manually pushed straight down to the center - here we have contact as shown by the meter.

So what do I make of this?

1) There is considerable slop in the mechanism.

2) There are very close tolerances on the pull length - I don't know how much this varies from unit to unit. Pull may be adversely affected by cocking due to mounting position, enough to prevent contact.

3) There are certainly manufacturing variations from unit to unit - you cannot make more than one of anything and avoid manufacturing variations from design specifications (tolerances).

4) Given the slop, pull length and manufacturing variations, it is entirely possible for a single unit to mess up even though there is no wear present. Depending on how the alignment of circumstances plays out on a single unit - one or most may work forever in any position you mount it in and another may fail.

5) I am speculating the manufacturer knows all of this and that is why they make the recommendation to mount the unit vertically - to improve the reliability rate.

For my part - hereinafter - all of my relays/solenoids get mounted with the plunger oriented vertically. While some or most units may work forever in other positions - it is evident that I ended up with one that was affected adversely by gravity due to the confluence of circumstances/tolerances particular to that unit.

Sincerely,

Dave S
KF7 Trigear
912ULS, Warp Drive