The L110 Lovejoy Coupling Dimensions I Ignored—and the $800 Lesson That Followed

I've spent seven years handling replacement-component orders for a small motion-control shop. I've personally made — and written down — four significant mistakes, totaling roughly $6,200 in wasted budget plus a fair amount of avoidable stress. This one was the worst because the part was sitting in my hand, and I still sent the wrong thing.

The day it happened

Last May, I had a quote for disc brake conversion kits open on one screen, an email asking what stepper motor torque a retrofit would need on another, and a phone call that pushed all of it aside. A packaging plant I'd worked with for years had a drive down.

The drive used an older Pacific Scientific servo motor connected to a gearbox through a Lovejoy coupling. The coupling's elastomer spider had finally let go after about fifteen years. The maintenance lead caught it before the hubs got badly damaged, but the line was stopped, and he needed a replacement fast.

He told me it was an L110, same as what was on there, and asked me to get the same Lovejoy coupling and a spider moving. That part number felt familiar enough. Too familiar, it turned out.

I had about two hours to catch next-day freight. Normally I would have sent someone over to measure both shafts and verify the keyway situation. But with the line down, I did the thing I warn other people not to do: I trusted a part number.

Fast thinking, wrong assumption

I pulled up the Lovejoy coupling catalog, found the L110 frame, and grabbed a set of hubs with a 1-7/8-inch bore from our shelf. That dimension came from the gearbox input shaft, which I had verified on an earlier rebuild. The motor side... I don't think I actually verified it that day.

Here's what I forgot: an L110 is a frame size, not a finished part number. When you search l110 lovejoy coupling dimensions, you get hub outside diameter, hub length, and bore range. But that bore range describes what the hub can be machined to, not what it already is. The old hub was stamped L110, but the motor-side bore had been finished to a metric size when the machine was originally assembled. The stamp didn't say that.

The two hubs I shipped arrived Friday afternoon. Monday morning, I got the call I still remember. The hub would not slide onto the servo shaft. The maintenance lead had measured the shaft with a caliper: 48.00 mm. The hub I sent was bored to 1.875 inches, which is 47.625 mm.

I stared at that number for a while. Actually, it was a text message, sent from the plant floor. That 0.375 mm difference doesn't sound like much, but when you're trying to fit a hub onto a motor shaft, it might as well be a mile. You can't force it, you can't politely tap it, and you definitely shouldn't try to run it.

Here's the part that still bothers me: the correct fix would have taken ten minutes during the original phone call. Ask for a caliper reading. Instead, the plant paid expedited freight for a bored-to-size L110 hub with a 48 mm bore and metric keyway. It arrived Wednesday, and they installed it that afternoon. The hub itself was maybe $85. The total extra cost was closer to $800 after freight and machine time. The lost production was more, much more. I don't like talking about that number.

What I do now before any L-series order

I keep a checklist taped to the edge of my monitor. It's not there because I enjoy checklists. It's there because I proved I need one.

1. Measure both shafts. Motor side and driven side, in two places each. Calipers, not eyeballs, not old drawings.
2. Read every marking on the old hubs. L110 tells you the frame. It does not tell you the bore. Look for a separate bore marking or a part number that includes one.
3. Write down the keyway. Width, depth, and whether it's metric or inch. This is easy to skip and hard to fix quickly.
4. Check the catalog for what you're actually ordering. The Lovejoy coupling catalog lists pilot-bore options, finished-bore options, and bore-to-size options. Frame dimensions alone do not complete the order.
5. Choose the insert based on the application, not the old part. A standard L-type spider may be fine for a general-purpose conveyor, but high shock, high temperature, or frequent reversing calls for a different material or durometer.

According to Lovejoy's own catalog data (lovejoy.com), an L-type jaw coupling hub is defined by its frame size and bore separately. You can order hubs with pilot bores or finished bores, and the finished bore is specified independently from the frame. I knew that. I did not apply it. The catalog was not the problem.

The honest part

I'm not writing this to say L110 couplings are tricky or bad. They're one of the most common general-purpose jaw couplings in industry, and for plenty of pumps, fans, and conveyors, they're exactly right. I still recommend them for most of those jobs.

But I've stopped saying that any L110 will work for any two shafts. It won't. And if you're putting a Lovejoy coupling on a high-cycle servo axis that needs precise positioning, I'll be straight with you: a standard L-type with a rubber spider may not be the best fit. It absorbs shock and tolerates misalignment, but it also has torsional windup. The motor might handle that fine, or it might fight the compliance all day. There is no single best coupling for every application. There is only the one that matches your speeds, loads, shaft sizes, and misalignment.

The ten-minute habit

Last month, the same plant needed an L110 hub again. I verified the shaft measurement first. I confirmed the keyway. I checked the old hub marking, then checked the bore spec a second time before approving the order.

It slid on first try. The maintenance lead texted me: that one went on like it should. That text is pinned next to my checklist now. The $800 mistake was dumb. The ten-minute habit that replaced it cost nothing. Prices from May 2024 are for reference only and vary with freight and distributor; verify current pricing before quoting your customer.