Drill Dissection What’s so interesting about a drill? It’s got lots of good stuff in it - electric motors, gears, bearings, etc It is a good example of an electro-mechanical power transmission system, and it is also an example of a machine tool (it drills holes) Instructor information: This is lecture material which is intended to supplement the laboratory dissection on a power hand drill. This material should take approximately 100 minutes to present (2 lecture sessions). The actual time will depend on how much discussion is allowed, how many physical examples are used, and how many “war stories” you tell. The first lecture can be done before the dissection, the second should follow the dissection so that relations can be made to what the students saw in their drills. The instructor should have a supply of actual hardware elements to pass around during the lecture. Readily available and portable elements include: ball or roller bearings, chains, sprockets, gears of all types, belts, pulleys, D’arsonal meter movement, digital multi-meter, etc.

Major Drill Components Gears (Transmission) Electric Motor Chuck Brushes Handle Bearings Trigger Switch

Bearings – Two Common Types Bushings - simple, cheap, limited life, porous material such as “Oilite” which holds oil like a sponge Rolling Element – support axial or radial loading, long life, grease or oil-filled, various types of seals, readily available from standard product catalogs, ex. ball bearings Discuss the different types of bearings seen in the drills. The drills we examined used many combinations of bushings, ball bearings, thrust ball bearings and needle bearings.

Roller Bearings Needle Roller Tapered Roller Spherical Roller

Rolling Element Bearing Parts Outer Race Inner Race The parts and nomenclature for a Ball Bearing Bore Ball Cage or Separator

Bearings -continued Hydrodynamic or Sleeve - oil filled, “no” wear, radial or thrust, common in automobile engines (e.g., crankshaft bearings) Rotating Shaft Sleeve Bearing Shaft Rides On Oil “Wedge” Oil Filled Cavity A typical radial clearance is on the order of .010”

Types of Gears Parallel Axis Gearing A - Spur gears, external contact B - Spur gears, internal contact C - Spur, Rack and pinion D - Spur, Helical gears E - Herringbone F - Pin gear ref. 4

Types of Gears - continued Non-parallel or non-intersecting axis gearing G - Bevel gears H - Spiral bevel J - Crown bevel K - Spiral gears L - Worm and wheel M - Hypoid gear ref. 4

Gears - continued a -Epicyclic train; the gear wheel C is fixed and the arm D moves around its axis at A, the wheel B will have a retrograde motion and the wheel A a faster motion in the direction of the moving arm; if the wheel A is fixed, B and C will have unequal forward motions. b -Sun and planet gear on a winch. c -Planetary motion as applied to an apple-paring machine. (a) (b) A D B G C A (c) (d) d -Epicyclic bevel gears; an arm FG is fast on a shaft AA ; bevel wheel B is loose on this arm; bevel wheels D and C are loose on the shaft AA ; differential motions of the arm FG around and with shaft A , or, by making the arm loose on the shaft, a differential motion of the shaft and arm can be obtained. ref. 4

Gear Transmissions Planetary Gearboxes - give high reduction (between Planet gears (4) Ring Gear (can be stationary) Sun Gear Planetary Gearboxes - give high reduction (between sun gear and planet carrier), compact size, can distribute or add power Planet Carrier

Gear Transmissions Two Stage, parallel shaft, helical gearset - Output Shaft Two Stage, parallel shaft, helical gearset - from the SKIL electric drill Ball Bearing Bushing Input Pinion

Electric Motors AC - most common type for power applications, simple, cheap, constant speed operation DC - easily controlled, variable speed operation, with or without brushes, also function as generator Stationary Windings (stator) Commutator Brushes (2) Armature (rotating unit) Internal view of the “Universal” motor used in the SKIL electric hand drill

General Types of Electric Motors Permanent Magnet DC Permanent magnet used in stationary field, rotor (armature) consists of windings, switching magnetic polarity to keep the motor turning, brushes make contact with commutator – only runs on DC and speed is proportional to input voltage Brushless DC Magnets are in the rotor, windings in the stator, polarity switching of the windings is done externally by additional circuitry, no brushes to wear out, no sparking or electrical noise –only run on DC; speed proportional to input voltage

General Types of Electric Motors Universal motor Same as a permanent magnet except wound coils (electro-magnet) replace the stationary permanent magnet; they will run on AC or DC and are most commonly found in electrical appliances as they are cheap and easy to vary speed AC Induction motor (Tesla’s genius) Wound coils in the stator, rotor has no magnets or coils, just a stack of steel laminations, speed is proportional to frequency of AC, making it difficult to vary speed of the motor

Universal Motor - AC and DC

Permanent Magnet DC Motor Operation Force generated F = (I x B) L B = magnetic flux density I = current in wire L = length of conductor + - Commutator (rotates with coil) F = force I Brush F B = magnetic field Permanent Magnet Permanent Magnet L N S B I = current Coil (single conductor shown) Build your own motor: http://www.exploratorium.edu/snacks/snackintro.html

How does a chuck work? Source: Macaulay, D., 1998, The New Way Things Work, Houghton Mifflin Company, Boston, MA.