Adjust Gear Mesh & Belt Tension

Topics like getting a proper gear mesh and good belt tension can be very confusing for new helicopter junkies. The term "feel" is used a lot when an experienced pilot is attempting to explain either of these procedures, and the problem is that a new heli pilot has nothing to base this "feel" on. Not to worry; with a little understanding and practice, setting gear mesh and belt tension can be made easy.

Gear Mesh. What is it?
Gear mesh is the relation between two or more gears where their teeth engage. The point where these gears join together is very crucial, as that point is where all power is transferred from one to the other. Basically, gear mesh measures how tightly two gears are pressed together as they operate.

Where would I worry about mesh in a helicopter?
There are numerous types of gears and locations in a model helicopter. Here is a short list of the most common:

• Pinion Gear: The gear that is attached to the motor shaft in an electric helicopter and the gear attached to the clutch assembly in a nitro one.
• Main Gear: This is the large gear found below the main shaft, receiving power from the pinion gear and driving the main rotor head.
• Tail Drive Gear: This gear is found underneath or on top of the main gear, and is driven by the main gear or by the main shaft via a one-way bearing. This gear can be of numerous types: a standard gear, a crown gear, or a belt pulley, depending on the helicopter it is in.
• Secondary Tail Gear: This gear is not found in all helicopters, but is present in many. When a helicopter's tail drive gear is a crown gear or a belt pulley, this gear is omitted. This gear meshes with the tail drive gear. This is usually done in one of two ways: In belt-driven tails, this gear shares a shaft with a belt pulley which will run the tail belt. Or, in shaft- driven tails, this gear shares the shaft with a bevel gear which will mesh with the bevel gear at the end of the tail shaft.
• Tailbox gears: In shaft-driven tail systems, the tailbox will have two bevel gears (one at the end of the drive shaft and one on the tail shaft) that mesh together to run the tail rotor.

Please note that in many modern helicopters, the mesh has already been set for some of these gears and the factory mounting holes are already set for them. In these cases, just bolt and fly!


Why is proper mesh important?
When any set of gears in a helicopter is not meshed properly, performance can greatly suffer. When gears are meshed too tightly, drag is induced in the system, which will rob power from the system, wear out the gears prematurely, and will put extra strain on the components that the gears are attached to (especially in electric systems, where the added strain on the motor can burn it or the power system out in a hurry). When gears are set too far apart, the teeth can strip in flight, there will be play between components, and the teeth will wear off quickly.

How to Spot & Avoid Gear Wear
There are a few clues that a heli will give you when a gear is wearing poorly. Look around a gear for large amounts of dust that is colored the same as the gear material, which would be actual material coming off the gear during flight. A little gear dust is normal during break in, but large amounts may point to a gear mesh that is far too tight. Also, look in between the gear teeth for marks and material which may indicate that the gears are meshed tight enough to bottom out on each other. When a gear is running too loose, the wear is generally indicated by the gear teeth rounding off. When a gear's teeth have lost their sharp edges you can be sure that the mesh is loose. Also, a loose mesh can often be indicated by missing teeth and slop between the components. All these types of wear can be avoided by setting the mesh properly and by regular checking of the gears to make sure they are still lined up.


How to Set Proper Mesh
So here it goes, there are a number of ways to set a good gear mesh. Car and truck drivers may have heard of the "paper method," in which a piece of notebook, printer, or even cigarette paper is cut into a small strip and placed between the two gears being pushed together. With the paper strip pressed tightly between the two gears, tighten the adjusting screws down and remove the paper. The paper should be pressed into a zigzag shape from the teeth, but not be cut through. If the paper is cut through, the mesh may be too tight. This method works for many applications and has been proven as a tried and true system. But it is important to check the gears regardless of the method used. To check gear mesh, hold the smaller gear of the two and move the other gear back and forth. A small amount of movement--very small--should be present. This movement is called backlash, the movement of the teeth of one gear inside the gaps of the other. This movement should, in most cases, be very small and just perceptible; anything more will be too loose. If no movement is present, the gears are too tight. This is where "feel" comes in. With time and practice, a pilot will know what the proper amount of backlash feels like. It is very helpful to have an experienced pilot set it the first time so that you can get a "feel" for it by moving the gears. Using the paper method will get very good results until this "feel" can be obtained.
It is also important to note that since most gears are not perfectly round, the mesh needs to be checked at various points around the gears. If there is a high point in a gear where the mesh is tighter than the rest of the circumference, it may work best to set the mesh tighter than normal at that point, so that mesh will be right around the rest of the gear, the high point should wear in over a short time.
All gears should be set with a small mesh, with as little backlash as possible (but with no drag or tightness). Another way to check if things are too tight is to just spin the gears and listen; they should spin freely with little noise. If there is a grinding noise when the gears are spun, the mesh may be too tight. Use shims or spacers wherever necessary to get gears nicely meshed together.


Belt Tension
Belts, where are they used?
Belts can be used in helicopters in two places: the tail drive and the main drive. Using a belt as a main drive (in place of a pinion and main gear) is pretty rare in a modern helicopter, but can still be found. Belt drives being used to run the tail rotor are more common, and in fact are becoming more popular than their shaft drive counterparts due to their ease of installation, lower parts count, and inexpensive repair costs. The belts in a helicopter have teeth in them as do the pulleys that drive them.


How to Spot & Avoid BELT Wear
Belts can wear just as gears do. Some telltale signs of a belt that is wearing out are a fine dust or powder coming from the belt, missing teeth on the belt, strands of the reinforcement bands coming out of the belt or stringing off of it, or teeth that are rounded on one side or the other. While it is difficult to avoid belt wear, as they do wear out quicker than gears usually do, it is possible to prolong their life by doing some of the following actions. Make sure that the belt is neither overly tight, nor overly loose. One way to check for a belt that is too loose is to grab the head block and tail hub at the same time, and while holding the tail hub tightly, turn the head block. The tail belt should not skip, even with considerable force. If it skips, tighten it up. Also, make sure the boom is straight and that the belt does nut rub against anything in its run. Once a belt has started rounding off at the teeth, it needs to be replaced.


Belt tension, how to set it?
Setting belt tension is fairly straightforward. The manual for the helicopter should have a general recommended tightness, usually measured by how far the belt can be pushed, or deflected, in before it stops. Start with the recommended setting. Setting this tension is simple; with a main drive belt, simply loosen the motor mounts and move the motor back and forth until the desired tightness is found (main drive belts like to be pretty tight to prevent skipping during use). With a tail belt, loosen the boom and pull it from the frame until the belt is at the desired tightness. Too tight of a setting will stretch the belt, rob power, and wear out quickly. Too loose of a setting can cause the belt to skip while running, strip teeth, rattle in the boom, and run off the pulley.

Why would I run the tail belt loose?
The tail belt can be run tight and loose, and unlike gear mesh, can be run either way and work fine (within limits). The main reason a pilot would want to run a relatively loose belt is to maintain power during autorotations. A tight tail belt will rob a bunch of head speed in a driven auto, making the auto more difficult to perform. A looser belt will use less power to run and creates less wear on the shaft bearings that support the pulleys.

Why Would I Run The Tail Belt Tight?
3D pilots generally run a tighter belt than sport pilots. The reason to run a tight belt is for greater tail authority. A tight belt has very little play and will not skip or flex during hard maneuvers.

Conclusion
Learning to set proper gear mesh and belt tension will help you throughout your time in this hobby. When the gears and belts are set correctly in a helicopter, the machine runs smoothly and efficiently, not to mention that the gears and belts will last a very long time as well. And who doesn't like that?