I cannot remove the string spool on the trimmer. When putting force in an attempt to turn the knob, the knob turns, but does not loosen. It feels like it is slipping.

https://www.youtube.com/watch?v=h_s4ThjQQcI
How Do I Remove a Ryobi Trimmer String Assembly?

1. Turn off the Ryobi trimmer. ...
2. Hold the spool of line in one hand and rotate the bump knob counterclockwise. ...
3. Remove the knob. ...
4. Let the trimmer come to a complete stop. ...
5. Locate the two opposite tabs on the sides of the spool retainer. ...
6. Pull up on the old spool to remove it.

https://www.hunker.com/13407105/how-do-i-remove-a-ryobi-trimmer-string-assembly

https://www.youtube.com/watch?v=zRSKP03VM1o

Sometime these get tighten more than you can hand turn and good large pair channel lock style pliers are required to loosen them.

One note here. I recently install a new trimmer on a customer's trimmer and the knob would not loosen. It jammed so tight I had to destroy it just to remove it. I found that the knob threads were damaged and new knob fixed the problem.

There should be a knob that goes on AFTER the string spool that holds it on. IF the issue is it keeps turning when you try to put it back on, then on a machine with no clutch, the engine will need to be locked up. To do this, remove the spark plug and wire, pull on the start cord until you see (with a light) the piston go DOWN in the cylinder...then insert a length of soft, clean rope into the cylinder and this will lock it up when it tries to turn again.Parts on end of the machine are:

1. Bump Knob 2. String Spool 3. Spool Spring and finally 4. String Head Housing

A typical trimmer has a small bump knob, an inner spool with the plastic string wound on it and the housing for that which is the string head.To remove the small bump knob, hold string head firmly and turn the bump knob counterclockwise. To remove the head entirely, first remove the bump knob. Next, remove the inner spool with string. What should be left is the spring with the end of the gear shaft showing. Some heads are locked on and some just slip over the end of the shaft. Check the gear drive just above the string head for any holes in the collar. An object can be placed in one of the holes to lock the drive and twist the head off.Ryobi's use retaining clips on the shaft which have to be removed.

There wiil be a nut or a knob at the end of the spool where string comes out turn it left to loosen and right to tighten it. Rmove spool pull down and spool will come off foll directional arrows that shows which way the string should go put spool back on tighten screw or some you can turn with knob at end make sure it's tight before starting.

• Turn off the trimmer and remove the battery
• Press the release tabs on the line spool cover and remove the cover by pulling it straight off. replacement spool is AF-100
• Lift the spool of nylon line out of the hub and clear any broken line and debris
• In case of jams and tangled line unwrap about one foot (30cm) of line to ensure that it is undamaged. If it is OK rewind it and insert the line end through the eyelet in the spool hub.
• Pull the line through the hole to maintain tension while placing the
spool down into the hub with the notched side exposed.
• Press the spool down GENTLY and rotate it until you feel it drop into place. (When in place, the spool will turn a few degrees left and right freely.)
• Snap the spool cover back on.

Had the manual but can't find where I downloaded it. Here's the link where I uploaded it
http://www.manualsonline.com/ex/thread/view/idThread/2062132

There sould be a knob that turns counterclockwise to take spool off. If not there sould be two taps that push in and pull down on what they call the spool head. Hopefully this will help you.

Or sometimes you unscrew a retaining bolt set into a plastic "lug", the spool drops out, refill t with line, replace it putting line through holes in the spool casing, refit the "lug" and tighten it again. I think I have the manual for this though and if so maybe I can send you a link to images on MSN Homepage to help you..

unscrew the knob on the bottom(turn cc wise) pull bobbin straight out . To restring pull start rope gently to see what direction it turns and wrap string in the opposite direction. Re insert the string into the corresponding hole(s) and screw up knob by hand until tight.