I'd say it's okay for some elbow movement to occur, especially if it involves getting a maximum bicep contraction. This is totally fine.
Now on the other hand, if you were having to heave the weight up with a lot of elbow movement... Ya know... But the way you are doing it is fine.
The better you can establish "muscle feel" during the exercise, the better your results will be.
Coincidentially, for something like hammer curls I'd recommend dumbbells over a pully. The only benefit to doing any biceps-specific exercise on a pully is that you get constant resistance, though usually with dumbells you can use more weight. Personally I'm not a big fan of doing biceps exercises using a pully. Every once in a while I may do some cable curls at the butt end of my workout but I mostly stick with dumbbell and barbell exercises for biceps. In my opinion, biceps exercises using cables would be more for refinement (bringing out shape) than mass or strentgh development so while cable curls may be good if you were training for a contest, you'd may do better choosing from other exercises...
My favorites include:Barbell curls (good for developing mass as this exercise allows for maximum weight)
Curl bar curls
Standing/Seated DB Curls
Incline DB Curls
Chin-ups
Preacher curls or one-arm DB preacher curls (occasionally)
Prone DB Curls (occasionally)
Concentration curls (occasionally)
** With DBs you can rotate the wrist as you curl the weight (palms facing towards each other at the bottom turning as you curl so that at the top you're pinky finger is higher than your thumb). This increases the amount of work the biceps are doing and for me really helped bring out my outer biceps so that when I flex you can see the outside head really stick out. Since I've really been focusing on wrist rotation during curls, that line has gotten more and more prominant (Keeping in mind I've stayed around the same overall mass, so it does help bring out shape even if you're not gaining a lot of additional mass).
Peace
