To expand on Jimbo's answer.
I've used them (Synchronous belts, Timing style belts) on a couple of TM designs, against my better judgement. Higher-ups in the company (TM Manufacturer) insisted we try them for the "cool factor". It was incorrectly reasoned that if they are OK for a drag car supercharger, then they would be equally well suited for a TM. Well, a drag car supercharger, even though a roots type design, is a completely different scenario. First, the engine on a drag car is eight cylinders and the blower is a helical lobe, and the engine has a harmonic balancer to boot. All contribute to a far lower torsional vibration potential with virtually no critical speeds. Cutting to the chase, the experiment with a two cylinder engine driving a traditional TM blower using a timing-type belt, was a failure. But it served to confirm all that we had accumulated in knowledge building direct drive coupled units.
The problem with the belts on a TM, is that the blower is a straight double or tri-lobe design. The operative word here is straight. Secondly, the engine used on a slide in is ordinarily no more than 4 cylinders, and sometimes a really nasty 60 degree v-twin. Both of these factors require a transmission device (belts, coupler, etc) to have a requisite torsional resiliency to absorb the vibrations created by both the uneven cyclical torque of an engine with such characteristics, and the uneven cyclical torque at the driven blower. These torque impulses occur at the point in a cycle wherein the engine is at max torque and minimum torque during each revolution. In essence, the engine's torque output looks like a sine wave that corresponds to the power output of the engine during a combustion cycle. The blower has a similar characteristic in inducing rotational (torsional) vibration during a single revolution. Both of these characteristics can be graphed and measured by the unit called the system "rotational inertia". The interaction of the rotation inertia for both the driven and the driver can be used to calculate the required torsional resiliency of the coupler.
Traditional v-belts have remarkable torsional resiliency, and can soak up quite a bit of the impulses without fatiguing and failure. Certain direct couplers also can often be used, if the "critical speed" is far away from any speeds the equipment will be used. Often flyweight itself can be used to balance a direct drive system. Where we see repeated failures of direct drive couplers is often where these factors have not been accounted for, and not due to the HP carrying capacity of the coupler itself. Often you see "newbies" make a direct drive machine, and have coupler problems. They seek to solve the problem by doubling or tripling the coupler HP size, and sometimes find it works. Unbeknownst to themselves is that the additional flyweight of the larger coupler "solved" the problem, not the capacity.
Here's a short primer on what torsional vibration is:
http://www.epi-eng.com/mechanical_engineering_basics/torsional_vibration_intro.htm
Anyway, to use "timing style" belts is way too stiff. You will find that if you use them singly as a drive component for a 2-4 cylinder engine driving a straight cut lobe blower, the result is very short life of the teeth on the belt, and often you get blower knocking too. Blower knocking is a phenomenon of too stiff a drive, as the high rotational vibration causes the blower lobes to be "jerked" rather harshly, wherein contact of one lobe is made with another.
Now, how is it that
HydraMaster can use a cogged timing-belt for the jack shaft on their
CDS machine? Simple really. The primary drive belt is a long poly-vee belt that "soaks up" or absorbs any interactive vibration of the driven and the driver components, alleviating any stress that would otherwise be transmitted, creating a very attenuated sine wave of vibration at certain speeds. Moreover, the engine that is used on a
CDS is much smoother running, with much lower differential torque peaks through a rotation. And the engine has a harmonic balancer. How many slide-in TM's have a harmonic balancer?
We also don't see any chain-drive units for the same reasons as a the timing belt won't work well on a 2-4 cylinder engine running a straight cut lobe blower.
Now, I see that that some manufacturers are dabbling with helical lobe blowers. This may provide an opportunity to revisit torsional stiff transmission components. As long as the torsional vibration is low enough for the belt's ability to absorb without quick fatiguing of the belt's materials of construction, it is possible that this type of belt can be economically feasible. Such feasibility includes the costs of harmonic balancing or additional flyweight as necessary.
