Technical Articles
Jaw Coupling Hub Cracked on a Concentrator Agitator? Loaded Starts, Hub Material and Bore Fit
A cracked hub or a broken jaw is a rarer failure on a Jaw Coupling than a worn spider, but it is the one that stops the drive outright. On concentrator agitators it tends to follow a pattern: the tank stops, the slurry settles, and the restart forces the coupling through a breakaway torque far above the running torque. Here is how to read the crack, which causes to check in which order, and when a steel hub is the right fix rather than another identical part.
Read the crack before you blame the hub
Where the crack starts tells you more than the fact that it cracked. Treat these patterns as clues and check them against the operating history, not as a verdict on their own:
| Crack location | Usually points to | Check next |
|---|---|---|
| Root of one or more jaws | Torque overload or jaw-on-jaw impact | Start history, spider condition, coupling size |
| Running axially from a keyway corner | Sharp keyway corners, a key forced in, or a loose key working | Keyway radius, key fit on sides and top |
| Through the hub wall from the bore | Shrink fit too tight or hub wall too thin for the bore | Bore and shaft measurements, fit used, bore against hub size |
| Starting at a set screw hole | Notch effect of the threaded hole | Hole position relative to keyway and jaw roots |
Look at the fracture face as well. A coarse, bright face with no markings points to a single overload, while curved bands spreading from one point show a fatigue crack that grew over many starts; either way, compare it with what the plant log says happened before the failure.
Loaded starts after the tank has settled
When an agitator stops with a full tank, solids settle around the impeller, and the restart needs a breakaway torque well above the normal running torque. A direct-on-line motor applies that torque in a sudden jolt, and if operators jog the motor repeatedly to free the impeller, the coupling sees a string of peak torques within a few minutes.
Each of those starts adds a load cycle at the jaw roots, so a hub that survives the first loaded start can crack after a few more. Motor current and start duration are useful clues to how hard the impeller was stuck, but read them together with the stop history rather than as a torque reading.
The spider matters here too. Once it is worn through, the jaws strike each other directly at every start and every load reversal, and a hub that has run metal-on-metal for weeks is often already cracked by the time anyone looks.
Cast iron or steel hub?
Cast iron hubs are common on standard jaw couplings because they machine well, damp vibration and cost less. Their weak point is shock: cast iron has little toughness, so under a hard loaded start it cracks rather than yielding, often without warning.
A steel hub, for example in C45 (≈ AISI 1045) or 42CrMo4 (≈ AISI 4140) after quenching and tempering, carries shock and keyway stresses far better. It is worth the extra cost when loaded starts cannot be avoided, when hubs have cracked before, or when the bore is large for the hub size; on a steady-running drive with a correct fit it brings little, and on an overloaded drive it only moves the weak point to the shaft, key or gearbox.
Shrink fit, keyways and set screw holes
A shrink fit that is too tight puts the hub wall in tension before any torque arrives, and cast iron is weakest in tension. Measure bore and shaft rather than trusting the drawing, keep the interference within what the hub material and wall thickness allow, and heat the hub evenly instead of with a torch on one spot.
Sharp keyway corners are where cracks like to start, so ask for a radius at the keyway bottom and check that the key fits on its sides with clearance on top. A key that bottoms in the hub keyway wedges the hub open, and a hub bored close to its maximum bore leaves a thin wall that carries the fit and the keyway load at the same time.
Set screw holes are notches too. Keep them where the drawing puts them, away from the jaw roots, and do not drill extra holes on site to stop a hub from walking; fix the fit instead.
Fixing it in the right order
Start by checking the size against the motor rated torque with a service factor that reflects loaded starts, and against the peak torque the starts really produce, compared with the maximum torque of the chosen size. Our LM range covers a nominal torque from 28 N·m to 14,000 N·m and a maximum torque of up to 25,200 N·m across its sizes, so read the figures for the specific size, not the range.
Then reduce the shock at the source: a soft starter or a frequency inverter lets the motor break the impeller free on a controlled ramp instead of a jolt, and a restart procedure that limits jogging helps as much as any coupling change. After that, keep the spider in service condition so the jaws never touch, and correct the fit and keyway before choosing hub material and heat treatment.
Typical scenario: an agitator on a leach tank stops during a power interruption, the slurry settles, and on restart the motor is jogged repeatedly before the impeller turns. If the hub later cracks at a jaw root and the spider turns out to have been worn through, the fix is a controlled start, regular spider checks and a hub that suits the duty, not simply an identical replacement.
FAQ
Can a cracked hub be repaired by welding?
No. Welding a cracked hub, cast iron or steel, leaves a hard, brittle zone exactly where the load is highest, and the crack usually comes back; replace the hub.
Should both hubs be replaced if only one is cracked?
Check the second hub for cracks at the jaw roots and keyway, with crack detection if you have it, and look for impact marks on its jaws. If it shows either, replace both hubs, and fit a new spider in any case.
Will a harder spider protect the hubs?
No. A harder spider carries more torque but passes shock to the hubs more directly, so on a drive with loaded starts it can make hub cracking more likely, not less.
Send us photos of the crack and the fracture face, bore and shaft measurements, keyway dimensions, the motor nameplate and starting method, and a short note on how often the agitator stops with a full tank. We make the LM Jaw Coupling from LM50 to LM400 to GB/T 5272-2017 with bores from 10 mm to 160 mm in Y, J or Z (1:10 taper) form to GB/T 3852, and replacement steel hubs in C45 (≈ AISI 1045) or 42CrMo4 (≈ AISI 4140) after quenching and tempering, with a material certificate (mill test certificate), heat treatment report and ultrasonic testing (UT) report available on request. Standard sizes ship in as little as 10 days and copies made from an old part take from 20 days, and non-standard hubs are confirmed after drawing review. Send us your drawing or the measured data, and we will review size, fit and hub material for your agitator.
Need an engineering selection or replacement review?
Send power, speed, shaft diameters, keyway, installation dimensions, drawings or old-part photos. We will review the working condition before confirming the model and quotation.