Coreless Motor - This refers to the armature of the motor. A conventional servo motor has a steel core armature wrapped with wire that spins inside the magnets. In a coreless design, the armature uses a thin wire mesh that forms a cup that spins around the outside of the magnets eliminating the heavy steel core. This design results in smoother operation and faster response time.
Indirect Drive - This refers to the potentiometer inside the servo. The final output shaft (the part that the horn/arm attaches) has to be supported not only near the end but also deep inside the servo case. Indirect drive is when the final output shaft is not dependent on the potentiometer for support inside the gear case. Normally a bushing or bearing supports the load. Direct Drive is when the potentiometer plays a supporting role in holding the output shaft in place. Most sub-micro servos are direct drive since they are tight on space and do not have the room for an extra bushing or bearing.
Spline - This is the output shaft of the servo. It is what you attach the servo horns or arms to. Standard Hitec splines are 24 tooth with standard Futaba splines 25 tooth.
Transit Time - This is the amount of time is takes for the servo to move a set amount, usually rated at 60 degrees. Example: A servo with at transit time of .19 sec. to 60 degrees would mean that is takes the servo nearly 1/5th of a second to rotate 60 degrees.
Torque - This is the maximum power the servo can produce. It is normally rated in inch-ounces. This means that the servo can move this set amount with a 1” arm attached to the output shaft or spline. Example: A servo with a torque rating of 130 in.-oz. can move that amount with a 1 inch arm or slightly over 8 lbs. To convert in-oz. to pounds of force, divide this rating by 16. Example: 130/16=8.125 which is in pounds.
3 or 5 Pole Motors- This refers to the commutator in the motor. The commutator is where the brushes make contact with the armature. The more motor poles the smoother and more accurate the servo will operate. Most servos have either 3 or 5 pole commutators.
Nylon Gears - Nylon gears are most common in servos. They are extremely smooth with little or no wear factors. They are also very lightweight. If your application calls for long duration but not jarring motion, nylon gears are a top choice.
Karbonite Gears - Karbonite gears are relatively new to the market. They offer almost 5 times the strength of nylon gears and also better wear resistance. Cycle times of well over 300,000 have been observed with these gears with virtually no wear. Servos with these gears are more expensive but what you get in durability is more than equaled.
Metal Gears - Metal gears have been around for sometime now. They offer unparalleled strength. With a metal output shaft, side-loads can be much greater. In applications that are jarred around, metal gears really shine. There are two cons to metal gears, weight and wear. First, metal gears are much heavier than both nylon and karbonite gears. Second, metal gears wear several times that of nylon gears. How quickly depends on the loads that you place on the servo. They will eventually develop a slight play or slop in the gear-train that will be transferred to the spline. It will not be much but accuracy will be lost at some point.