Putnam Planer Motor Drive Sizing: Belt and Pulley Configuration

Overview

The Putnam planer is a reciprocating metal-cutting (and historically woodworking) machine produced in the 18-inch and larger classes by the Putnam Machine Company of New Haven, Connecticut. Restoring one of these machines is largely a question of re-establishing a proper mechanical power transmission chain from a modern electric motor to the planer's main bull gear. Unlike a CNC retrofit, a planer of this vintage expects a long flat belt, a belt shifter, and a jack shaft (also called a counter shaft) to step the motor speed down to a useful cutting stroke rate.

This reference consolidates the field-proven layout used by hobbyists and small-job machinists: a 2 HP to 3 HP single-phase or three-phase motor mounted overhead, an overhead jack shaft with tight/loose pulleys for start-stop, and a flat belt descending to the planer's side shaft. Specific values are given for a 40-inch platen with 16-inch clearance between uprights, which is a typical Putnam 18" class machine.

Machine Specifications at a Glance

Drive System Architecture

The classical Putnam drive is a two-stage flat-belt system:

1. Motor pulley (small, fast) on the motor shaft.
2. Jack shaft (counter shaft) suspended from the ceiling or from a frame above the planer. Carries a large driven pulley and a smaller pulley on the opposite end, plus tight/loose pulleys for the belt shifter.
3. Planer side shaft driven by a flat belt from the jack shaft. The side shaft carries the original "S" spoke pulley and feeds the internal gearing through a belt shifter mechanism.

Motor Sizing Calculation

Planers are intermittent-duty machines, but cuts on cast iron with carbides can draw sustained load. Use the following for a first-pass estimate:

Required motor power:

Where:

• F_cut = cutting force in pounds (typical 200-500 lb for a 1/8" depth of cut on mild steel with HSS)
• V_cut = cutting speed in feet per minute (typical 30-60 fpm for finishing, 20-30 fpm for roughing)
• eta = overall drive efficiency (flat belt 0.95, gear train 0.90 per reduction, bearings 0.99 per pair)

For a typical finishing cut on mild steel with a 1/8" depth of cut:

• F_cut = 300 lb, V_cut = 40 fpm
• P_cut = (300 × 40) / 33,000 = 0.36 HP at the tool

Multiplying by a 1.5 service factor for intermittent peak loads and 0.85 combined drive efficiency yields a nameplate requirement of about 0.65 HP. However, planer installations universally oversize the motor to 2-3 HP because:

• Start-up under loaded belt is severe
• Large-diameter flat belt pulleys have high rotational inertia
• The original line-shaft-driven planer was always connected to a much larger prime mover

Jack Shaft and Pulley Selection

The jack shaft (counter shaft) is the heart of the retrofit. The motor end carries a small pulley (typically 2"-3" diameter) and the planer end carries a larger pulley (typically 6"-10" diameter) sized to deliver a side-shaft speed in the 150-300 rpm range.

Belt Selection

Two belt widths appear in original Putnam installations:

• 1-1/2 in wide belt — factory-original for many 18" Putnam planers. Marginal for sustained cuts above 1.5 HP.
• 2 in wide belt — preferred upgrade. Drives a 2-3 HP motor with margin and resists slip during peak cuts.

Use modern polyurethane flat belting with a friction surface rated for use with cast iron or steel pulleys. Leather is acceptable but requires periodic dressing. Avoid neoprene-impregnated belts that glaze quickly on bare wood or cast pulleys.

Belt length calculation:

Where C is the center distance between shafts, D is the large pulley diameter, and d is the small pulley diameter. For a typical overhead installation with the motor 8 ft above the planer side shaft:

• C = 96 in, D = 9 in, d = 3 in
• L = 192 + 18.85 + 0.1875 = ~211 in

Order belt stock 5-10% longer than calculated and trim to fit the lacing or clipper.

Belt Shifter Linkage

The belt shifter moves the upper belt between a tight pulley (driving) and a loose pulley (freewheeling) to start and stop the planer without stopping the motor. On older Putnams, the shifter linkage is a forged rod running parallel to the jack shaft with a forked end engaging the belt.

Inspection points during restoration:

1. Verify the shift rod is straight and free of nicks at the fork.
2. Confirm the fork end is square to the belt face (not twisted).
3. Check that the rod's lateral throw is sufficient to move the belt fully onto the tight pulley without contact with the loose pulley crown.
4. Inspect the bell crank or lever that the operator pulls. Worn pivot pins cause sloppy shifts and belt slip.
5. Confirm spring tension is present; most shifters return to "loose" by spring when the lever is released for safety.

S-Pulley Versus Straight-Spoke Pulley

Putnam originally shipped the 18" class planer with small-diameter S-spoke pulleys on the side shaft. These S-spokes shift the belt direction through a 180-degree wrap on a small pulley, but they have a notorious weakness: the belt slips easily on the small diameter, especially under peak load.

Field fix: replace the S-spoke pulleys with larger straight-spoke pulleys (8"-10" diameter) and modify the shift rod to clear the new geometry. Larger pulleys:

• Increase belt wrap angle
• Reduce belt slip
• Reduce side shaft bearing wear from side-load belt tension

When modifying the shift rod, use a tubing bender to replicate the original forged bend geometry. A misaligned shift rod will not engage the belt cleanly and will accelerate belt wear.

Power Feed Mechanism

The feed rod on a Putnam planer drives the cross-slide automatically across the platen, allowing the operator to make a series of parallel cuts without re-setting the tool by hand. It is engaged by a small clutch lever on the apron of the cross slide.

Feed engagement procedure:

1. Bring the platen to the back (return) position with the belt shifter in loose.
2. Disengage the feed clutch.
3. Position the tool at the start of the cut.
4. Engage the feed clutch.
5. Shift the belt to tight; the platen will advance through the cut while feeding the tool.
6. Shift to loose at end of stroke, return platen, repeat.

The feed rod is particularly useful for planing angles on a fixture and for "straight down" cuts where the workpiece geometry prevents turning the part on the table.

Wiring and Motor Control

For a 2-3 HP single-phase motor, use a NEMA motor starter with thermal overload, a local disconnect, and a magnetic starter with 120 V coil. A simple wiring diagram follows:

L1 ---+---[Disconnect]---+---[Starter Coil]--- N
        |                  |
        +---[OL Heater]----+---[Motor]--- N

For 3-phase:

L1 ---+---[Disconnect]---+---[Contactor L1]---[OL1]--- Motor T1
L2 ---+---[Disconnect]---+---[Contactor L2]---[OL2]--- Motor T2
L3 ---+---[Disconnect]---+---[Contactor L3]---[OL3]--- Motor T3

Wire the forward/reverse contactor (if equipped) to swap any two phases. Add a mechanical interlock to prevent both contactors energizing simultaneously.

Commissioning Steps

1. Static checks: Rotate the side shaft by hand. Confirm no binding and that the bull gear, feed clutch, and shifter all operate freely.
2. Belt tension: Apply 8-12 lb of mid-span pressure on the upper belt. Deflection should be approximately 1" per 10 ft of center distance.
3. Pulley alignment: Use a straightedge across both pulleys. Misalignment greater than 1/16" per foot of center distance will throw the belt.
4. No-load run: Energize the motor with the belt shifter in loose. Verify correct rotation (typically counter-clockwise viewed from the side shaft end). Listen for bearing noise.
5. Loaded run: Shift to tight. The platen should begin reciprocation. Watch the belt for tracking; minor lateral drift is normal.
6. Cut test: Plane a piece of mild steel bar 1" × 4" with a 0.020" depth of cut. Confirm no belt slip and that the motor draws less than its full-load current.
7. Thermal check: After 15 minutes of intermittent cutting, feel the motor housing. Warm is acceptable; too hot to hold is not.

Maintenance Schedule

Troubleshooting Matrix

Recommended Reading

Two texts cover planer operation in useful depth for the restorer:

• Machine Shop Practice, Volume 2, by Karl Moltrecht — practical chapters on planers and shapers with sketches of typical setups.
• Treatise on Planers, Cincinnati Planer Company, 3rd edition (1944) — heavily illustrated, useful for "miniaturizing" production techniques to a small machine.

Older training manuals from the Ford Trade School also document gang tooling arrangements for surfacing engine lathe beds in a single setup.