A cam shedding mechanism gives each heddle frame its own cam on a shaft that turns once per weave repeat, so for plain weave the cam shaft runs at half the crank speed and for a four-pick twill at a quarter. Each cam must hold the shed open, its dwell, for at least 20-30 degrees of its own turn in plain weave, and must lift against the pull of the warp, about 93 N per frame of 1,000 ends on assumed figures.
Part of Power Loom Construction
Using cams on a timed shaft to raise and lower heddle frames automatically, and how to dimension them.
The Cam Shaft Turns Once Per Weave Repeat
The overview says to mount two to four cams on the crankshaft, each controlling one heddle frame through a lever, with springs or weights to return the frames to the down position; for plain weave two cams 180° apart, for twill four cams at 90° intervals. The cams are timed from the crankshaft, but they cannot be on the shaft that turns once per pick. A plain weave repeats every two picks: a frame is up for one pick and down for the next, so its cam must complete one turn in two picks. A cam on the crankshaft, turning once per pick, would lift each frame on every pick. The cams therefore sit on a second shaft geared down from the crankshaft: 2:1 for plain weave, 4:1 for a twill that repeats every four picks, and so on. Read the overview’s “on the crankshaft” as “driven in time with the crankshaft”. Pattern repeats of more than four frames use the dobby (Dobby Mechanism Basics).
The Dwell Must Cover The Shuttle’s Flight
From Weaving Mechanics: the shed must stay open for at least 40-60° of crank turn at 100 picks a minute. For plain weave the cam shaft turns half as far, so the dwell is 40 ÷ 2 = 20° to 60 ÷ 2 = 30° of cam rotation. For a four-pick repeat the cam shaft turns a quarter as far: 10-15°. These are minima; add margin for the shuttle entering the shed and clearing the warp, and set the timing by turning the loom by hand with a shuttle in the race.
The Cam Lifts Against The Warp
On the shed geometry of Shedding (front length 300 mm, back length 400 mm, lift 40 mm) an end held at tension T adds a vertical pull T × h × (1/300 + 1/400) = T × 40 × 0.00583 = 0.233 T. With an assumed T = 0.4 N per end, each end pulls with 0.093 N, and a frame of 1,000 ends pulls 93 N, about 9.5 kg-force. A frame lifted against the warp needs that force from its cam; a frame being lowered is helped by it. The inertia is far smaller. For a simple harmonic rise of 40 mm over 60° of cam rotation (assumed) at 50 rpm (ω = 5.24 rad/s), the peak acceleration is (h ÷ 2)(π ÷ β)² ω² = 0.02 × 9.0 × 27.4 = 4.9 m/s², and a 2 kg frame (assumed) adds only 2 × 4.9 = 9.9 N. So spring or weight return forces can be modest, and the warp supplies most of the load; the cam surface wears where the load is high, at the start of the lift.
What You Need
- A cam shaft geared 2:1 (or 4:1) from the crankshaft (Gear Ratios).
- Cams cut from hardened steel or hardwood faced with iron; profiles are turned and filed (Turning Operations for eccentric and cam work).
- Followers and levers to the frames, with a lever ratio set by the lift needed.
- Return springs or weights.
- Lubrication (Lubrication).
Method: Laying Out And Timing The Cams
Step 1: Decide the repeat and so the cam shaft ratio.
Step 2: Draw the cam profile: rise, dwell and fall, with the dwell from the minimum above plus margin.
Step 3: Fix the lever ratio so the cam rise gives the frame lift (Shedding): a 2:1 lever turns a 40 mm cam rise into 80 mm of frame travel.
Step 4: Cut and fit the cams at the angles the weave needs. For plain weave the two cams are 180° apart on the cam shaft, which at half speed is 360° of crank turn, one pick apart.
Step 5: Time by hand, turning the crank slowly, with a shuttle in the race.
Step 6: Run slowly, then at speed, watching the followers for chatter.
Worked Example
Plain weave at 100 picks a minute: the crank turns at 100 rpm and the cam shaft at 50 rpm, a 2:1 reduction. Each cam turns once in 1.2 s, during which the frame is up for one pick and down for the next. If the pick is 0.6 s and the shuttle needs 0.10 s, the cam dwell must cover 0.10 ÷ 1.2 × 360° = 30° of its turn, matching the figure above.
Common Mistakes
- Cams at crank speed, lifting the frames every pick.
- A dwell cut to the minimum, so any lag closes the shed on the shuttle.
- No lubrication, so cam and follower seize.
- Return springs too strong, which add to the cam’s load.
Safety
Cams and levers close on whatever is between them
Followers, levers and the cam shaft gears move on every pick with a force of tens of newtons that does not stop for a hand. Guard the cams and gears, adjust only on a stopped, locked loom, and know where the stop is and who can reach the gate (Power Drive System). Children keep away from running looms (Task Assignment by Age).
What’s Next
- Learn next: Dobby Mechanism Basics — pattern shedding beyond four frames
- Combine with: Heddle Construction — the heddles the frames carry