Top rollers must press on the bottom rollers with 5-15 N per cm of roller face, which is 15-45 N, about 1.5-4.6 kg-force, on a boss 3 cm wide. Apply it with lead saddle weights, springs or lever and weight systems, matching the same load at every spindle, because a nip that is too light lets fibers slip and one that is too heavy crushes them.
Part of Spinning Frame
Applying and adjusting the pressure that holds fibers between top and bottom rollers.
Pressure Per Centimetre Is The Quantity To Control
The overview says top rollers must press hard enough to grip fibers but not so hard as to crush them, gives a typical nip pressure of 5-15 N per cm of roller face width, and names three means: dead weight (lead saddles on the top roller bearings), springs (adjustable, more consistent), and lever systems with weights.
The load on one boss is the pressure per centimetre times its width. For a boss of 3 cm (an assumed width): 5 × 3 = 15 N to 15 × 3 = 45 N, and dividing by 9.81 m/s² gives 1.5 to 4.6 kg-force. The same pressure applied along a full-length 56 cm roller (8 spindles at 70 mm, assumed) would need 5 × 56 = 280 N to 15 × 56 = 840 N, which is 29-86 kg-force, and would sag the bottom roller (Fluted Bottom Rollers). Separate weighted bosses avoid both problems.
The Three Means Compared
- Dead weight (lead saddle). A mass rests on the top roller bearing. Its load is exactly its weight and does not change as the leather wears. It cannot be adjusted without changing the lead, and saddles can bounce on an uneven roller.
- Spring. An adjustable screw sets the compression, so loads can be trimmed spindle by spindle. A spring’s load changes with deflection: with 10 mm of compression, 0.5 mm of leather wear changes the load by 0.5 ÷ 10 = 5%. The overview calls springs more consistent; this arithmetic does not reproduce that for wear, though springs are easier to trim and to raise off the roller.
- Lever and weight. A weight W hung at distance b from the pivot, with the roller bearing at distance a, loads the roller with W × b ÷ a. To get 30 N from a 1 kg weight (9.81 N), b ÷ a = 30 ÷ 9.81 = 3.06. A lever lets one hanging weight serve several rollers and lift off together when the frame stops.
What You Need
- Lead or iron weights cast to a known mass, or springs wound from spring steel.
- A spring balance to check each boss’s load.
- Hooks, saddles and levers forged or cast to fit the roller bearings.
Method: Setting The Load
Step 1: Choose the load per cm within 5-15 N, starting at the low end for weak or short fiber, and compute the load per boss.
Step 2: Make the weights to that load; weigh each on the balance.
Step 3: Fit and check. With each roller seated, pull a paper strip from the nip with a spring balance (Leather-Covered Top Rollers).
Step 4: Run and adjust up if fibers slip at the front pair, down if the leather crushes or the yarn looks flattened.
Step 5: Lift the weights off whenever the frame stands for a long time.
Worked Example
A 1 kg lead weight is 9.81 N. Hung at 90 mm from the pivot on a lever whose roller bearing sits at 30 mm gives 9.81 × 3 = 29 N, within the 15-45 N range for a 3 cm boss. If the lead is cast to 1.00 kg ± 5%, the load is 29.4 N ± 1.5 N, so a batch of weights should be checked on a balance before use.
Common Mistakes
- Using the same load for all three pairs without testing, though the back pair usually needs a different grip from the front.
- Casting weights by eye. A scatter of 10% is an uneven yarn.
- Weights left on an idle frame, which set the leather.
Safety
Lead is poisonous, and the weighted nip draws in fingers
Lead fumes and dust damage the body, especially in children and pregnancy (recalled general fact, not from the corpus). Cast it outdoors or in strong ventilation, never in a space used for food, wash hands before eating, and keep children away from the casting. The weighted roller is a nip that pulls in fingers; lift weights off and stop the frame before touching it (Drive System).
What’s Next
- Learn next: Multi-Spindle Frame Assembly — putting many weighted spindles in a row
- Combine with: Roller Spacing and Gauging — the gap at each nip line