Re: Front Hex wheels advantage??
Posted: Wed Mar 10, 2021 2:39 pm
I've done a fair amount of plastic part design with my work and I assumed the general shift from in-wheel bearings to hexs had a lot to do with manufacturability and, by extension, cost.
Getting a good, consistent press fit requires good tolerance control. Pretty easy in metal, though it can get costly. In plastic, we've done them, but general molders will balk at a tolerances down around +/-.001in, which you need for consistent fits. Gear molders will do that without much fuss, but they're usually specialized molding houses, work with parts they are usually small and simple enough for a straight-pull mold, and sometimes use multiple "gates" to keep the pressure consistent across the part if it's a really high quality tooth profile.
*I just looked at my old Losi LXT/Pro SE slipper gears and they're triple-gated. An old Robinson Racing spur I can't readily see the gate. Might be edge-gated in the hub. But I have CA glue from an old project obscuring it. I want to say these gears wobble in their application, anyway.
A small hole and single-sided hex feature with a more typical +/-0.005 (or possibly more) would be a lot less costly to mold, and more consistent, than two bearing seats that also need to be lined up. It may also use less plastic to counter hoop stresses, be an easier process to control, and you'd not have as much of a quality assurance job. This all adds up to cost savings.
Of course, the wheel hub now has these features and they're plastic, but they're already made more stout and with a better fiber reinforced material (if not metal) than the wheels, in lower quantities, and are building onto an existing close tolerance shaft-mating feature. So, even if they shift, let's say, $0.10 from the wheel to the hub, that's $0.10 they pocket for each set of wheels that go on those hubs.
Getting a good, consistent press fit requires good tolerance control. Pretty easy in metal, though it can get costly. In plastic, we've done them, but general molders will balk at a tolerances down around +/-.001in, which you need for consistent fits. Gear molders will do that without much fuss, but they're usually specialized molding houses, work with parts they are usually small and simple enough for a straight-pull mold, and sometimes use multiple "gates" to keep the pressure consistent across the part if it's a really high quality tooth profile.
*I just looked at my old Losi LXT/Pro SE slipper gears and they're triple-gated. An old Robinson Racing spur I can't readily see the gate. Might be edge-gated in the hub. But I have CA glue from an old project obscuring it. I want to say these gears wobble in their application, anyway.
A small hole and single-sided hex feature with a more typical +/-0.005 (or possibly more) would be a lot less costly to mold, and more consistent, than two bearing seats that also need to be lined up. It may also use less plastic to counter hoop stresses, be an easier process to control, and you'd not have as much of a quality assurance job. This all adds up to cost savings.
Of course, the wheel hub now has these features and they're plastic, but they're already made more stout and with a better fiber reinforced material (if not metal) than the wheels, in lower quantities, and are building onto an existing close tolerance shaft-mating feature. So, even if they shift, let's say, $0.10 from the wheel to the hub, that's $0.10 they pocket for each set of wheels that go on those hubs.