Bartack Sewing Machine

Overview

A bartack sewing machine sews a short, dense pattern of stitches called a bartack at points on a garment that take repeated load. A bartack is a tight group of zig-zag stitches laid across a narrow zone and then locked, so it concentrates many stitches into a small reinforcement. Common counts are 28 or 42 stitches in one tack, though the count and the tack length are set on the control. The places a bartack goes are familiar from any pair of jeans or work trousers: the ends of belt loops, the corners of pockets, the base of the fly, the button stay, and the ends of zipper openings. These are the spots where a plain seam would pull apart first, and the bartack holds them.

The machine combines a single-needle lockstitch head with a programmed feed. The Needle Bar and Thread Path drives the needle up and down while the X-Y Feed Mechanism shifts the clamped fabric a small step between stitches, so the stitches land where the pattern says rather than in a straight line. The Rotary Hook and Bobbin and the Needle together form a Rotary Hook based lockstitch. The whole cycle, from clamp down to trim, runs from the Control Box and Interface once the operator presses the pedal.

Forming the stitch

Each stitch is a 301 lockstitch, the same stitch a straight-lockstitch machine makes. The Needle carries the top thread down through the fabric. As the needle rises a small amount it throws a loop of thread to one side, and the Rotary Hook, turning at twice needle speed on the Hook Shaft, catches that loop and carries it around the Bobbin held in the Bobbin Case. The loop passes around the lower thread, and the Take-up Lever then pulls the slack up so the two threads lock in the middle of the fabric. Top thread tension comes from the Thread Tension, where the Tension Disc pair squeezes the thread and the Check Spring takes up slack as the needle enters. Even tension on top and bottom is what keeps the lock buried inside the cloth instead of showing on either face.

The X-Y programmed feed

What makes a bartack machine different from a plain lockstitch head is how the work moves. The fabric is pinned under the Work Clamp and Lift, which clamps it to the Feed Frame. Between each needle penetration the feed shifts the clamp a programmed distance in two axes. On an electronic machine this is done by the X-Axis Stepper and the Y-Axis Stepper, two stepper drives that move the X Slide and Y Slide by exact steps the control commands. The needle is up while the feed moves and down while it is still, so each stitch lands at a defined point. Run the stitches close together across a short line and then lay a row of cross stitches over them, and the result is the dense tack. Older mechanical machines did the same job with a Feed Cam and an interchangeable Feed Plate, where the cam profile fixed the pattern and changing the tack meant changing the plate. The electronic feed replaces that with memory: the pattern is a list of X-Y moves the Control Board reads from its stored programs, so the same head can sew a 28-stitch tack, a 42-stitch tack, or a custom size by selecting it on the Operator Panel.

Drive and timing

The needle, hook and feed all run from one main drive. A direct-drive Servo Motor turns the Main Shaft with no clutch and no separate motor belt, and a Position Encoder tells the control exactly where the needle is so it can stop with the needle up or down. The shaft drives the Needle Bar through the Needle Bar Crank and turns the hook through the Timing Belt and Hook Timing Gear. The Lubrication System keeps the hook and bearings fed with oil from a sump through the Oil Pump and felt Oil Wick wicks, which matters at the high speeds the hook reaches.

Trim and cycle end

When the tack is finished the machine cuts both threads and clears the tail so the next piece can go straight in. The Thread Trimmer and Wiper swings the Moving Knife out under the throat plate to catch the top and bobbin threads and shear them against the Fixed Knife. The Thread Wiper then sweeps the top thread tail clear of the needle, and the Work Clamp and Lift lifts on its Pneumatic Cylinder or solenoid so the operator can pull the part out. The two-stage Foot Pedals runs this: the first step lowers the clamp, the second starts the sew cycle.

Applications

Bartack machines are standard equipment in jeans and workwear plants, where every pair has bartacks at the belt loops, pocket corners and fly. They also tack the ends of zippers, the corners of flaps, the joins on bag straps and webbing, and the load points on safety harnesses and slings, where the tack carries real force. Because the pattern is stored and the feed is exact, every tack on a run is the same, which is why the machine is built around the programmed X-Y Feed Mechanism rather than the operator guiding the work by hand.