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Small dc motors like the one below are commonly used for various tasks in toys and small appliances.

small dc motor

These motors almost exclusively have perfectly cylindrical round shafts without any notches that have an almost polished appearance. The products making use of these motors usually have small plastic gears or corkscrews attached to these shafts which are held by friction alone.
These friction fit parts are usually the first thing to fail in these products which can usually be heard by the motor running on maximum rpm seemingly without making contact with the gear any more.

Why don't these motors have a notch in their shaft to ensure a safe connection?

Busti
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    At a guess, the shaft will have a plastic (or if you are lucky, nylon) gear press-fitted onto it. Having it round like this means that if something "jams up" (kid holds the wheels so they can't turn, for example) the shaft can slip without seriously breaking something (like the cheap, plastic gear.) Might have similar safety concerns for small appliances -- though I know little to nothing about the legal environment and how it impacts design/utility questions. – jonk Nov 12 '20 at 00:53
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    @jonk the motor could just stall rather having the gear slip. – D Duck Nov 12 '20 at 01:11
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    D-shafts cause enormous forces in the corners that can easily break the hub in plastic gears. Source: repair of a 1979 RCA VCR bought new for approximately $1,800. – Spehro Pefhany Nov 12 '20 at 01:13
  • @DDuck Yeah, that too. And also Spehro's comment, as well. (I had considered Spehro's thought for a moment but "things got mentally complicated then" so I just didn't bother writing it. Spehro has specific experience to inform his comment. So I'm glad he added it.) Altogether, we may have a more complete picture of what an answer might look like. – jonk Nov 12 '20 at 01:15
  • Also - if it's a small shaft, how do you grind the flat and how do you make the D, the key, or the grub screw part on the gear? I think the problem with the D is the small radii on the corners. The other problem is that small gears seem to be made out of some sort of magic "goes brittle with time" plastic. – D Duck Nov 12 '20 at 01:20
  • 1. Minimal cost of overall solutions. If a press fit works it's cheap. If it's over engineered them slim it down. 2. Note that vastly higher power RC / skateboard / ... BLDC motors may also use smooth shafts. A Collet will allow solid attachment and allows tailoring the solution to the standard production motor . 2. Geared down the motor can make substantial force.in even low power applications so may not be suitable for protection of user. Stalling may not occur. – Russell McMahon Nov 12 '20 at 02:55
  • Loctite!!! :-) ................ – Russell McMahon Nov 12 '20 at 02:56
  • I’m voting to close this question because it has nothing to do with *electrical* engineering being rather about the *mechanical* issues such as concentricity and stress, and the economics of manufacturing. – Chris Stratton Nov 12 '20 at 03:06
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    @RussellMcMahon Loctite dissolves a lot of plastics. – DKNguyen Nov 12 '20 at 04:29

2 Answers2

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'D' shafts are not appropriate for plastic gears because the sharp edges cut into the gear hole and chew it out or break the gear apart, whereas a round shaft with slightly larger diameter will fit tightly and not damage it (provided a good quality plastic is used).

'D' shafts are used on 1/10th scale '540' size R/C car motors, combined with a steel or aluminium pinion gear and grubscrew. This is required due to the high torque load which is too much for a plastic gear, and greater convenience when separating them compared to a pressed on metal gear (which needs a gear puller to remove).

They are also commonly used in small appliances such as hand drills and electric screwdrivers, often coupled to a molded metal pinion with matching flat. In this case the pinion is simply slipped on and held in place by the gearbox, so is easier to assemble. The down side is that a larger 'flat' is required to avoid chewing out the gear hole, which weakens the shaft. This technique is also sometimes used in devices such as hair dryers and vaccuum cleaners which have a plastic impeller, with a metal clamping ring added around the outside to make a stronger connection and prevent the plastic from splitting.

Another downside of the 'D' shape is that the coupling may not be perfectly concentric. For applications that need good balance, such as model airplane propellers, a Collet adapter is often preferred. This clamps tightly around around the shaft while ensuring that the output shaft is precisely centered.

enter image description here

In reality small electric motors are almost always manufactured with round shafts, then ground down when necessary to produce the 'D' shape. Motors sold for general purpose use don't have a flat because the customer might not want it, and they can always grind or file a flat into the shaft if they need it.

Bruce Abbott
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  • You could consider adding a key + keyway method for completeness. – Russell McMahon Nov 12 '20 at 10:49
  • Keyways are generally used only on larger industrial DC and AC motors where the shaft is large enough to take it. – Bruce Abbott Nov 12 '20 at 12:22
  • That makes sense. My best guess was that the high rotation speeds of these small motors might generate enough friction heat that, in the event of a failure, the gear might re-attach to the shaft that way. – Busti Nov 18 '20 at 03:02
  • If a plastic gear slips it's usually toast. But these small DC motors generally have low torque so a well fitting gear shouldn't slip. Sometimes the manufacturer splines the end of the shaft after the motor is assembled (won't fit through the bearing if done before), which makes a stronger connection - but wreaks the gear if it does slip. – Bruce Abbott Nov 18 '20 at 05:37
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Because it saves a manufacturing step and so keeps the cost down.

In most small applications the motor is designed to run at high speed loaded with very little torque (because that's how to run a motor efficiently) and therefore a press fit (for either plastic or metallic gears) is more than good enough.

I'm curious what you're doing where you're seeing multiple failures. Perhaps if you're re-using scrap...

Beyond press fit, you can:

  1. use loctite as suggested in comments
  2. As Russell McMahon suggests, use a collet to increase the holding force
  3. enhance the press fit by heating the (metal) gear before pressing it on; it shrinks as it cools giving a tighter fit (interference fit)
  4. roughen the shaft : a straight knurl approximates a spline
  5. Then grind or mill out a D, or a keyway, or a set of splines in roughly increasing order of strength.

But a properly designed press fit for the application should hold perfectly well.

  • I have recently had this failure occur in an electric pencil sharpener. I fixed the issue by carefully applying a small droplet of superglue to the shaft, while trying to avoid locking up the motor and then re-attaching the gear. When I was younger I remember seeing this failure in multiple toys, but I cannot remember what toys they were specifically. – Busti Nov 18 '20 at 03:04